In the short term-replace him. In the long term send him to a vocal coach.

As a private self education approach, the biggest problem most vocalists (and many instrumentalists have) is pitch recognition/matching. Use a pitch generator and have him practice five minutes a day matching pitches to the drone-unisons, 5ths, 4ths, M3, m3, M6, m6, M7 etc. Approach the intervals from both above and below. Trust me, you'll know immediately when the interval is correct.

Secondarily, have him practice whistling EXACTLY in pitch with some ballads and then moving on to quicker tunes. Gershwin is great for this even if you hate that style of music.

You will be surprised-provided the singer has any pitch recognition ability at all-how quickly this will help someone whether it's a singer or sax or bone. There is no help for trumpets or bagpipes though each have their place.....

Not the answer you were hoping for but-

what is his problem?

if it's pitch, it is possible to pitch correct in a way that is 100% transparent (transparent meaning undetectable). this obviously depends on how bad his pitch is. what you need to do is invest some time in using the graphical mode on auto tune or better yet, look into some other pitch correction software: Melodyne by Celemony, Tune by Waves or Variaudio (comes bundled with Cubase 5).

obviously, there are lots of problems we encounter as engineers and they tend to come in bunches. Melodyne fixes timing in addition to pitch (as does Variaudio which I use quite a bit). you can also fix timing with your editing tools: splices, x-fades and time-stretches.

for tone (sound quality of the voice) EQ is critical. a little hi-pass filtering is usually a pretty good place to start. with bands added as needed to sculpt the vocal

obviously, dynamics (the consistency and control of your singer's volume level) are hugely important to a good sounding mix. the way you control these is learning to use compression. there are many types of compression, but the idea behind a basic compressor is evening out the overall loudness of your track. this is one of the most commonly misused effects, so be careful, but if used properly will help the vocal out tremendously.

if your singer is also having problems with sibilance (overpronounced "s" and "t" sounds), you may need to apply a de-esser. a de-esser does what it says it tames the "s" and "t" sounds to create a smoother, more natural vocal.

lastly, your "taste" effects can add that little extra something: the subtle addition of reverb, delay, chorus, whatever works in context with the music.

There is truth in what DefiningFreq writes.

However, for my money I'd rather have an actual musician to record instead of a bozo. 10 minutes a day of quality vocal practice will be better in the long run.

One thing I have learned in my recording career:

HIRE PROFESSIONALS - IT IS WORTH THE MONEY.

Otherwise you try to get a friend or someone who knows nothing and you think you're saving money but the 10 hours you just spent getting a mediocre recording from someone and the 5 minutes it takes a professional......

You do the math - even with like a 50$ an hour studio.

ahhh this posting is something else......
a great producer will do a few things to take care of this problem.
1. You must know the mechanics of the voice in order to try to produce someone in singing.
2. find the strong parts of the persons voice and build upon them and eliminate the weak ones.
3. you may need to change the key of the song to fit their range properly.
4. Lyrics and their cooperation with the music plays a big part of the way a person is going to sing. there are just some words that dont belong in a song.
5. are they singing to loud almost yelling? singing should be no more or just a touch louder than your speaking voice, its like driving a car in first gear at 90 miles per hour sure you'll get somewhere, it wont be pretty and there will be damage.
6. Voice lessons for years
7. NO AMOUNT OF PITCH CORRECTION IS GOING TO HELP IF THE ABOVE ITEMS ARENT ADDRESSED.
8. FIRE THEM.
i know it hurts to tell someone hey man, you cant sing. and if you keeps singing wrong like this your never going to get better.
i know everyone wants to sing, its a great feeling to sing along with your favorites, but without proper education you will NEVER get there. vocals are a muscle, train them wrong (such as no training at all) and you risk damage or they get "stuck" in one position. i have been singing for 33 years. i've had 6 voice teachers since i was 13. i was fortunate that the guys that mentored me in music at 13 saw that i had a voice but said "just like im teaching you bass guitar you need voice lessons if you also want to be a singer.
i am proud to say that when producing a vocal, thru my extensive education and experience, i can produce a vocal for someone that has a really hard time singing and make them become a singer, to a point. i teach them the proper way to produce sound, what they are doing wrong, encourage them to seek a professional teacher, and i can produce a killer vocal for someone that can do the job of singing, i can push them beyond their belief of where their voice can go, i can extract true emotion from them in regard to their lyrical content. so i say to you hondacrxdude13, if YOU dont posses the abilites outlined above you are only making it worse for the possible singer.
THAT'S WHAT A PRODUCER DOES

I agree with Planet10s 8 points whole heartedly. I would also like to stress 2 of them:

2. find the strong parts of the persons voice and build upon them and eliminate the weak ones.
3. you may need to change the key of the song to fit their range properly.

Even though Honda has probably already recorded his friend or ditched him for a new one, I think that for anyone trying to just record a friend for fun or what not should keep the previous points at the forefront.

One of the number one things I've found when recording vocalists (my main instrument) is that they come to me with a song or songs they have written and they key of the song is not right for their voice. Example is they are still trying to sing 80's hair band falsetto stuff and their strength of their voice is better for more Barry White! With friends I tell them up front, with people I don't know so well or clients you have to feel them out and let them know where you hear their strengths and what key. It really is a producer job, but the lines get blurred nowadays when there is one person behind the board maintaining everything.

Something everyone is overlooking (and I've found this in other posts as well) is budget.

Planet10s suggestions are always spot on, but sometimes he misses the fact that many of us have no budget for some of these things:
Hired musicians, a producer, etc. Or not working w/ a less-than-stellar band.
Some of us simply have to make the most of what we've got.

I started out recording friends for free and gear trade.
Now I charge a low, per-project rate (not hourly).
Yes, I waste a lot of time on struggling musicians.
Yes, I could be more selective.

Unfortunately, this is something I'm building on the side, so I deal w/ it.
I'd rather have a band in that forces me to overcome their inadequacies, than none at all.
That's no way to improve your skills, or run a business.

Everyone has made at least one suggestion you can make use of.
Another is to coach a little (producing w/ a softer side):
Have them do a couple of takes, and make a note of the better and worst passages of each. Play these back and point out the strong and weak points.
It's important that you give some sort of constructive feedback ("you did well on verse #2 in the 1st take..."), or the artist can shut down.

As long as you're working w/ lesser talent/gear/space, you'll never get the take you want.
But if you have to work w/ lesser talent/gear/space, you can make use of some of the above techniques and learn and get something you can live with.
Trust me, I am more critical of everything I touch than everyone else combined.
But I also know the place I'm in, and that I have to make my own lemonade.

@soapy
with or without a budget, my job is to make them better. i would rather do it for free, the job of teaching someone how to sing better than push them away and say comeback when your ready. they've heard that probably too many times.
in my producing tactics, im about sensitivity in what im telling them, they are singers, (the MOST sensitive of performers) and one wrong statement and YOUR cooked.
to be honest, i am fully aware of the budgets that dont exist with most bands nowadays. i still give of myself 100% above what they have asked me to do for them. i want musicians and performers to be better educated in the process of what i do, what i do for them, and what i can do for them after the project is finished. i believe that everyone deserves a shot at being heard by the masses, its my job to make them be great at what they have. far too many times i get demos or full lengths of bands asking me to produce them, i go see them and they sound nothing like their recordings. i am proud that my clients sound great on their recordings and in the venues because i keep them grounded and at the same time creative. that is something that the newbies of our business ( and im not pointing connectors at anyone here ) FAIL to provide musicians. seems that its about the engineer and not the band.
soapy.....it doesnt take anything but one on one communication with an artist to make them better. the gear, the facility, has nothing to do with being a great producer. your statement of lesser talent/gear/space is bull, you'll only get the take you want ,regardless of your mentioned fails, until you communicate properly. my advise in the previous post is what i learned from the real producers i worked with such as Barry Fasman, Michael Freeman, Danny Leake.

planet10-

I'm sorry if I miscommunicated.
Having read your most recent post, I agree w/ your approach -
Even though we operate under different circumstances, it's exactly the same as mine.

My comment wasn't directed specifically at you.
I did mis-understand that you were suggesting the hiring of a producer.
Something that many of us can't fathom.
My point was that some advice lately, >generally< has been to spend/refuse money the OP clearly doesn't have.
I've seen a rash of "your gear is crap / your clients suck / your space is inferior" posts, some of which I may have mistakenly attributed to you. In general, these posts are extremely valid (like the headphone thread), but overlook the actual budget of the OP (not necessarily the case in the headphone thread).
Maybe the problem is the original posts are in the "Pro Audio" forum?

As an example to my point, I'd LOVE to have a producer present so I could focus more on the engineering side of things. Like you described, sometimes getting the job done as an engineer or "studio" involves donning the producer's hat. More often than not.
Which was the initial reason for my post (hondacrxdude) - to say that many of the previous advice was valid, but that if you can't get a vocal coach, producer, new mic, preamp, or room treatment...
sometimes you have to go beyond what you think is necessary to get the job done, using only the "crap" you have before you.

It's all about ending up w/ a product you and the band are happy with, and leaving them w/ a good experience along the way. Do that, and customers will keep calling.

soapy
cool.....

Depending on the plug-in you are using, you can do some pretty drastic pitch correction without it being noticeable. What are you planning on using?

Is your friend planning on singing live at any point... because this could pose a problem if he can't sing. In this case, a vocal coach would be highly recommended.