Be warned. The InfoWindows aren't your average 'dumb terminals'. They are, what is known in IBM speak as Fixed Function Workstations. They have some (probably rom-based) intelligence built-in that you would need to emulate.

There used to be an IBM product called something like InfoWindow Control Program, a DOS program that allowed a pc to be used to talk to the InfoWindow system. It also exposed an API. There may have been a unix version or similar. I can't find much reference to the program on the first page of a google search, but if you (your friend) contacted IBM, they maybe able to offer specs or even give you access to source -- I've known them do that for withdrawn products, though they'll probably try and sell you an 'upgrade':)

If not, you may need to resort to using a line monitor to dump a session and reverse engineer the protocol.