Please don't leave her alone with him.
My Best Friend(we've been friends for 36 years next month)was married to a really horrible bastard and I hated him and so did my DH and her then shit bag husband knew it.
One evening he smashed up her apartment and assaulted her.
My DH is a Police Officer,he went straight round the apartment,we sat with her whilst she made her statements and we helped her clean her place up.
Honestly it was a miracle her didn't kill her,he'd kicked her and pushed her down a flight of concrete stairs!
Every time she wanted to talk I listened,I got smart and learned not to slag him off to her face,so if they made up she wouldn't remember what I'd said and worry about telling me that they were back together again.They're relationship was very up and down and on and off because of him.
I worked on her,I helped build her confidence up,she spent a lot of time with us and our 5DC and I got her back in touch with our old friends and eventually a guy that used to like her.
She had already kicked him out when the assault happened.He was supposed to be picking his stuff up,she thought she'd be safe,she wasn't but thank God she survived the attack.
She got an injunction against him and divorced him.She dated the other guy for a little while and had some fun.Now she's getting married again this time to an EX Marine that is so protective of her and would never lay a finger on her.If your friend means as much to you as mine does to me,play the long game OP,when she's left the bastard and is happy again she will thank you for always being there for her.