How can someone go about taking advantage of a girl with AS and get away with it?

I suggest you research autism because I don't think the situation would arise. Autistic people usually like order and routine and any deviation from that is not something that they would appreciate or even accept. That is my experience of autistic children. It may be that your story would fit, although I doubt it, but it is always a good idea to find out for sure. Good luck with your story.