My ex and I saw, at the top of the street, on a night out in Brighton, a huge bloke smack his girlfriend in the face and the edge of the kerb knocked her out when she landed (really made my stomach go, that did!). He was still going at her, kicking her, so my ex managed to pin him down until the Police got there and I tried to bring her around whilst waiting for the ambulance. They carted him off, but she refused any treatment and just wanted to go home and wait for him in the end!
It wouldn't stop me intervening again, though. I'm with 4get, too if you smack someone first (man-woman, woman-man), you're asking for a smack back.