It's a very sensitive situation, and I think the advice offered so far is sound.
It would be easy to wade in with a few mates and kick the shit out of him, but that can be seen as making it more about you than it his her. Also, these types are basically cowards, and will hit back at the female in various underhand ways. Scratched cars, broken windows, character assassination, vile rumours etc...
Personally, I'd run everything to do with this situation past her. The last thing she needs is another male taking things into their hands and making decisions for her. Decisions that may well compound her problems further. By all means support her, and if she asks for opinion, then it's fine to give it, but she needs to take the lead on this.
This fella clearly has serious psychological problems. I imagine he is insecure and immature, thus controlling in order to try to claw some perception of power back in some way. People like that cannot accept rejection. I've seen it a million times, and so has my partner. She had a marriage that was similar. I've actually had a female partner who was physically and emotionally abusive too.
It seems this lady has a number of things in place to try to protect her, and I assume the police are aware too? Maybe it's best to let this scumbag hang himself with his own actions? The fella will relish anything that causes distress for this lady, so if you butt in then he could turn it around on you and get you into trouble. This causes further disruption in her life, thus makes him happy that he's got such influence.
He needs to feel like he still has a controlling stake in her life, so don't fall into the trap of giving him a stick to beat you (and indirectly, her) with by you being aggressive or even violent towards him. As I said, he's looking to hold any power and influence over this lady and her life as he can get. He'd love to report any partner/friend of hers to the police so he can look the innocent party whilst also relishing the fact he's messing with her life all the more. The clever, long game is probably the best approach.