She fucked around and found out.

I was bullied at school for a long time. I told my bully to stop, but of course he didn't. He even bullied me during class and I couldn't pay attention even if I wanted to. One day I snapped and punched him in the face. Only once, but hard. He ran out of the classroom with a bloody nose, his best friend had to mop the floor in the corridor. He wasn't seriously hurt. No one was mad about the incident, not even the teacher. They knew and understood what I went through. The bullying stopped and we got along quite well after that.

I hate violence and this was a last resort, but some people have to learn the hard way.