I'll admit my first reaction was to wade in and smack the little turd into next week. If a bystander hadn't told me the police were already on their way, I may just have.
Thing is with those situations, you never know the full story. I mean, was the attack unprovoked? What were the circumstances leading up to it? was the guy justifiably angry about something, and just expressing it in completely the wrong way?
Alternatively, was everything exactly as it seemed at first glance - some waste-of-space chav tearing strips out of his completely innocent girlfriend? You could literally go mad thinking about it.