If he is physically threatening her, I think she needs to get him to a counselor..... both of them.......she also needs to keep something near her at all times to protect herself if he comes at her................other than that, I agree, she needs to take control........fast and swiftly...........And she needs to be pretty tough on him for a month or so and then if she thinks she can, let up a bit..........what I mean by that, have a talk with him......let him know the new rules, you do this, this happens.........and for how long.....in other words, you yell at me, you lose TV for a night, you yell again, 2 days.....etc.....no discussion, no compromise..............
Yes, she needs to find out what hits him......going out to play, being on the baseball team, etc.........then take it away...........make sure she puts this information out where they can both read it.....so that if he does something, he knows pretty much what the punishment is going to be. (And in case she forgets, it's there for her to go to and point it out to him) Then she has to follow through and it doesn't matter how much begging he does, he is NOT off the hook.........
She can do this........and if this doesn't work, tell her that she can have him put into counseling.
Tell her Good Luck, stay strong, and to hang in there......