Lisa,
It is NEVER your responsibility to make a decision for a student -- with or without disability -- that you impose on that student because YOU believe it is in their best interest. Unless you can tell me with a straight face that you have never enrolled a student with a drinking problem, or who was enthusiastic but inept, or any other possible impediment to success, you certainly did the right thing in not standing in the way of a student with a disability whose potential you doubted because of what you knew about her disability.
Dr. Jane Jarrow