My friend slapped his girlfriend after she slapped him-- was he justified?

Anonymous
My friend and his girlfriend got into a fight in public, and she slapped him in the face. He immediately slapped her back. She was stunned for a few seconds, then started crying. Her friends came to her and immediately started cursing/ screaming at my friend.

Some people have stopped talking to him and others are badmouthing him behind his back, calling him a woman-beater etc. I told him he was justified and that didn't change my opinion of him, but people are upset with me for sticking with him. Even my own sister is angry at me.

Was what he did wrong?
My friend slapped his girlfriend after she slapped him-- was he justified?
22 Opinion