How strong are you & how much do you care? A neighbour of ours was once in a situation like yours; they had kids.
I used to go over during the escalation & confront the husband. I never said anything directly, just let my presence be felt. This let them know what was going on wasn't okay & that we all knew about it. He hated me for it, but was scared of me also. My husband ran a kind of 'silent cover' (he was there, supporting, but silent). In calmer moments, he spoke w/the husband & let him know things weren't cool. We decided those roles b/c the gal needed to see a 'strong female' & to avoid the males just beating on each other.
In some ways, I think it escalated things, but only temporarily. One day, when my son wasn't home but I was, she came running over (it was awful, I could hear the kids screaming next door). I let her in, called the cops, & told him he'd better leave asap when he showed, which he did.
She left him w/in the week. Its been hard, I still hear from her occassionally, but at least their kids are safe & they aren't in an environment w/all that fighting going on.
My point is, some women will gain the strength to leave when they are given a strong example & understand they need to go, esp to keep someone else safe, like their kids. You should have this kind of talk w/her. And also make sure she has the info re: women's shelters, I also gave this info to our neighbour.
Second thoughts can generally be amended with judicious action; injudicious actions can seldom be recovered with second thoughts.
--Cyteen by C.J.Cherryh