I think we're making this too complicated. There's no doubt in my mind that he should be able to use the M-Audio USB MIDI keyboard in conjuction with the Tascam interface with no problems. As long as he's using ASIO drivers for the TASCAM and there are USB drivers for the M-Audio keyboard, there should be no conflict. The M-Audio has no onboard sound module so the MIDI data would be unidirectional.
Like I said, I have a Roland PC-300 set up this way, used in conjunction with my sound cards, and there are zero conflicts. All that is required is to select the M-Audio keyboard as the input device. MIDI is very simple data that is difficult to screw up and slow down. It will keep up with even the lowest latencies, no doubt.