I really appreciate these replies - each of you is saying what I think, which is why I'm conflicted!
Yes, she's a relation of mine. We were both brought up in violent families. I didn't realise until my thirties that it wasn't normal. I don't think she ever has - she says she does, but her actions & reactions belie the words. My guess is that, as a young girl, she saw her mother being hit and decided "I'll never put up with that". But she also saw her siblings getting it and, as a child, thought "That's what happens to kids."
In my recollections, she was severely abused but was also the golden child (how fucked up is that?) She now remembers only verbal abuse, not the rest. So I suspect she's normalised everything except the shouting. I am, as some of you have said, having trouble accepting the enormity of what she is.
I hadn't thought that she'll stay after her H turns on her. I see that's quite likely :(
I have told her DCs, regularly, in front of her, that what they're going through is wrong. Unfortunately my words are just a drop in the ocean of their lives - they are wonderful kids, but have predictably troubled relationships. This is what gets me down most about my family - the cycles are already re-starting. Her eldest is going to uni this year, the others have got a couple more years at home.
Part of me feels I ought to stay friends with her, as at least she's got someone to talk to about it and, at least, I know where she's coming from. The other part feels I'm heading towards serious cognitive dissonance and should just tell her, then dump her 
They ALL told SS they were okay and I'd exaggerated a bad row.
Thank you so much for replying! I completely understand the anger in some of your posts; I need to hear it as well as the balanced perspective.