I was in a far more "fortunate" situation when I faced abuse. I had no kids, nothing that in my mind tied me to that man more than the vows we'd made.
We had always had a physical relationship, lots of wrestling, horsing around, etc., but I'd never been struck at in the way I was that day. And it was just that once. I walked out.
He got counseling, did all the right things, but I couldn't go back. We've each (happily) remarried, and I wish him all the best, but I couldn't stay.
That being said, I truly believe that there are times when the assault is... for lack of a better term, a fluke. The shock I saw on his face when he realized he had struck at me in anger, and the efforts he made to see both a man of God and a therapist, told me he was serious about healing whatever had broken inside him that day. For me, however, the risk wasn't worth it. Had we had children, it might have been different, but I doubt it.
Praying for your friend, and for you. It's a hard path to travel, and I don't think there are ever black and white answers.