I agree with every word that famous said, but I'm going to add one thing: Helping the woman is not the same thing as busting the man. The moral question, therefore, isn't "Should I call the cops on this guy?" but "What can/should I do to help?" In your particular circumstance, your courses of action were constrained by the circumstances, but those constraints have little to do with the moral question, which is what you asked about.

I think if I had completely free reign about what I did, I would speak with the woman and try to help her see that she DOES need help. That's a monumental task all its own, but in so doing, you are (1) helping someone who does not want the help, and (2) not jumping to make decisions for her.

Like famous, though, I wouldn't hesitate to call the police if a situation got violent, even over her own protests. Public violence is more than sufficient cause for arrest. If she wants to fight it, she can refuse to press charges, and then the police won't have a choice but to let him go. But I'd do what I can to talk to her in the meantime, if she'd let me at all. The bottom line is that while you can do some things, you can't truly help anybody in the long term unless they want to be helped. This woman NEEDS the help, whether she knows it or not, and so the first real step, beyond any necessary temporary measures, has to be to get her to see that.