Kicking the shit out of him is a big risk in my opinion. He might turn violent against her if he gets frustrated and that's the worst thing that can happen.
It's tricky, man. Maybe you could try and get closer to her, you know, just hang out with them and especially her a lot. Don't try to pick a fight with him, just ignore whatever he does. Then after some time when he does something out of line again try to talk to her, to make her see that he's no good. Beating him up will accomplish nothing, it will only draw her closer to him. I think she has to realize it herself and there's nothing wrong with helping her realize it.