Depends on the girl. Some want their honor defended. Some want you to defuse the situation peacefully. Some would just prefer that I sit back and let them handle it. I’d just do whatever I think she’d have wanted me to do then ask her about it later.
I actually grabbed a girl’s butt in front of her boyfriend once. Not my finest moment. He punched me in the nose with enough force to make it bleed but not break it. Nobody believed that I’d did it, and they made him leave the bar. Several years later we made amends. I found out he went easy on me. He could have easily broken my nose. He’s a guy you don’t want to cross. My saving grace was that I didn’t mouth off to him or try to fight back.
People do crappy things to other people. Sometimes they happen to you, sometimes you are the one doing them. The best thing anybody should do in any situation is the thing they feel is right. Talk it over later after everyone cools off.