The first thing you should do is make her understand that the guy she's with doesn't deserve her. Once she understands that she shouldn't be with him, she will deliberately leave him. You won't need to step in unless he gets clingy and doesn't leave her be. But I doubt it'll get that far if you can make her realize he's not right for her. You don't need to tell her you want to be her bf for her to leave him but just say that you deserve better. She will really appreciate you for being there for her, even if you don't become her new boyfriend afterwards. She's most likely dating this jerk to fill the gap you left when you didn't want to be with her. My mom is doing the same thing with her boyfriend. You need to show her that someone still cares about her.