This is a tough situation, basically it all depends on how you feel about it.
If it is really bothering you and you don't think you can get past it, maybe it is worth ending it. But speak to your boyfriend seriously about it, she is telling people he forced himself on her but that might be false.
Everyone does things in their past they regret.
If she went to the police there is a chance he could be taken in for it, but without evidence and just being hear-say there's not much they will be able to do.