Excellent question. I've had this thought many times as to what I would do.
I've seen this happen twice in my life. A dude hitting a girl out in public. Once I was in a carload full of guys at a gas station and we pull up and some Persian or Armenian looking guy is roughing up his girlfriend. One of my friends rolled down the window and told him to break it off. Seeing that there were like 5 of us and one of him, he wasn't about to do anything stupid after that.
Next time was driving with my sister, we saw some dude hitting and roughing up a woman in this rundown part of town that seemed deserted. I drove by and my sister was like, "you should do something" and I turned the corner and debated it. Thinking that I could end up getting shot over some sh-t that isn't even my fvcking business to begin with. When you step into a domestic quarrel, you are really playing with fire and there's no telling what the hell the girl did to make the guy that mad, she could have have just fvcked his best friend for all you know and here you come along trying to play hero taking her defense. At the same time, you would feel like a wussbag for seeing a man beat on a woman in public and not doing anything to stop it. So anyway, I decided after turning to corner to go back and investigate so I went around the block and came back down that street and no longer saw them. So I just left.
It's a tough position to be in, and doubly tough if you are with a girl. If you're with a woman, you basically can't do nothing without looking beta.
On top of that, let's say you are the kind of dude that would intervene, well would you do it if the dude was big and ripped?