Calling the cops sounds reasonable, but it's probably not the best idea. I mean for them to arrest him she has to say that he does whatever to her. If she's not willing to the call the cops herself, how likely is she to tell the cops he does these things if you call them? Then if she doesn't report him but denies it, then he's going to be wondering quite a bit who called the cops, why they called the cops, what she has told others about him, and so forth. That's pretty bad cocktail.
is she a crazy nagger and liar behind closed doors? You don't even know if this guy has ever even actually touched her. Let's assume he is as bad as is said about him. Well you got two paths, you can either get to him and try to get him to change, and best case scenario is he does and they live happily ever after. The other option if it were me, and I saw no hope to try to convert the man, then you have to try to work on the girl and convince her to leave him, in this case, I would think tell her parents, and let them make the judgement.