This topic is full of sexist posts. It's nauseating frankly, as someone who was physically assaulted by my first wife (she was convicted of assault, it's on record). Society has a disgusting, hypocritical view of female-on-male violence.
First of all, no one should be hitting anyone. Men shouldn't hit men, women shouldn't hit women, men shouldn't hit women, women shouldn't hit men.
Once you throw a punch at someone, they can defend themselves. Period. She initiated the violence by throwing the first punch. His retaliation was warranted, because she may have followed that first punch with another punch, or the alleged knee to the groin that supposedly did occur. Now, if he had knocked her down, and followed her to the ground, kicking and punching, then that goes beyond self defense.
White-knighting has no place here. Again, she initiated the violence. If some 120 pound weakling of a guy punches me in the face without warning, I'm probably going to hit him back even though I'm much larger. And the size gap here is not nearly as large as it would be between me and a 120 pound guy. She's probably close to his body weight.