Wow, that's a toughie. I would want to speak with the girl's doctor and find out if keyboarding with the one hand that has limited use is even healthy first. So many of us with two working hands are accumulating damage from keyboarding, it would be bad to have the same happen to her.
You might check these folks out
Keytools - Provides mice, keyboards, and input devices for education, special needs, RSI and disabilities. http://www.keytools.com/index.asp
There are companies that make special needs keyboards that are also for people who can only use one hand. You might find a company that would donate one in return for a promotional plug on your part.
Chances are you're going to have to learn a new way of keyboarding to teach this student as well. Might be a nice specialty for you in the end.