What to do if a girl loves you but has a boyfriend?

It isn't your place to judge. She has obviously chosen to be with him for some reason or other. Be very careful getting involved in such a situation as it could get ugly. You could be the one physically hurt through all this. Keep a good eye on things just like you're doing and tough it out. Hopefully if things get worse she will have the sense to back out of the relationship and you can be there to help her out. She's probably trying to change him. Tell her gently, but firmly that it isn't going to happen. In any event, tell her that she deserves better, and that you care about her too much to see her be treated that way. There's no reason for her to stay with someone like that.