It's a bird!
It's a plane!
No, it's Timmy Tuffnuts!
Man, never a shortage of these dudes just looking for an excuse to unload on another guy.
Just pick and choose your fights carefully. That dude was wrong headbutting the woman. She may have provoked him, but he could have chosen to behave like an adult human instead of a neanderthal (no offense to nss if he's reading this).
None of us like to see a woman get abused. Ironically, some of the same men who say this clap and cheer when some two hundred eighty pound freak is trynna smash another man half his size. Nobody sticks up for the little man.
Back to the woman. If you would have beaten up Hillbilly Jim, in which case I would not have felt sorry for him, then chances are the story ending would not have been any happier.
Hillbilly has a weapon, or Hillbilly's friends show up, or the police show up, or the girl suddenly takes HIS side for God knows what reason, or Hillbilly loses the battle but plans to beat, rape, or murder the girl at a convenient time in the future when you are not around; he now has DOUBLE the incentive to hurt her, for one he's angry at her for prior stuff and now he's humiliated by your intervention. You now have a new enemy, or heaven forbid you get hurt.
About ten years ago, there was a woman who was murdered in the middle of the street, beaten to death as dozens of people looked on. They could have stopped her from getting hurt and I wish they did. It would have been the right thing. By intervening, they could only make the situation better for her. Unfortunately, in these situations, nobody does anything. However, when some coward headbutts a woman and runs away, suddenly everyone becomes a superhero. The pvssy ran away! Let him go! What's more important? Getting the girl to a doctor and making sure he stays away or trynna show her how big your shlong is by beating this dude (who's probably her boyfriend) senseless while she continues to bleed all over the place? What if this guy was much, MUCH bigger than you, knocked her out, and then got right in your face and asked you if you wanted to be next? Not too many guys would stand toe to toe with him then. No, it's much more convenient to play tough guy when you have little chance of getting hurt and you think you're gonna save a girl or win her over. Hardly ever works out that way. For all you know, that girl and Hillbilly could have been in love and she may not have appreciated your efforts.
You did the right thing, doing nothing but make sure she was going to get help.