Is there any excuse for this?

I think you are right about this woman. She sounds like she's just using him because she knows how he feels about her. The only problem is that it's his life and if he wants to let her do this, it's his decision, he's a grown adult. I understand you are concerned about how this is going to affect him once she gets tired of playing with him but in life, you have to take the ups with the downs. You had to learn from your mistakes and so does he. Just because he's in a wheel chair, you can't protect him from everything. You can keep an eye on the situation, as a friend, and if things start getting out of hand, you can stress your concerns to him or to her. If you interfer too much, you could be the looser in this situation. If you cause the end of this thing between them, he will blame you and maybe down deep he really knows what she's up to and don't care. Give him the benefit of the doubt, now, to make his own decisions, but let him know as a friend that you are concerned of what the outcome may be. He needs a friend not a mom.