If he hit her I would intervene if I felt it was safe, most likely I'd ask others for help or call the police.
If he was threatening I would ask her if she was OK. Someone once asked me that when I'd had a huge argument with my boyfriend and refused to walk with him, so it looked like he was stalking me. I really appreciated it, even though I didn't need the help, but my boyfriend didn't
I find that women can get away with a whole lot more than men in such situations (ie intervening)