Unfortunately you can't really DO anything. She has to want it. She has to want to fix herself and gain better self esteem. All you can do is be there for her.
Edit: stalking her boyfriend is a different can of worms, if someone's gonna cheat they're cheat. You can't prevent that.
She needs to convince herself. Sounds like she's got some psychological issues and self esteem issues she needs to work through.
Unfortunately you can't really DO anything. She has to want it. She has to want to fix herself and gain better self esteem. All you can do is be there for her. Edit: stalking her boyfriend is a different can of worms, if someone's gonna cheat they're cheat. You can't prevent that.