holy fuck just imagining it hurts the fk out of me. Because i was in your shoes before and know how it feels to have a bully.
You gotta realize that she went to him because he is more dominant than you and even dominated you. Girls are attracted to dominant guys cause they find them attractive and safer with them. You gotta realize this and admit it, then start working on yourself and getting stronger. Don't let anyone bully you anymore no matter how scared you are. You gotta fight them even if they will kick the fk out of your ass. you just be determined and fight everytime then with time you'l get used to it and won't be scared from anyone (literally). I still remember the day i had to stand infront of the guy who bullied me and faced him, i almost shyt my pants but i knew i was strong cause i worked hard on myself. It was all just fear in the end, after i fought with him and almost got my ass kicked (luckily teacher stopped us) he stopped fighting me and bothering me again. because he noticed i can hit hard.
Just trust me man, if you want a better life. work hard on yourself and never let FEAR control you. Instead, you control your fear and face it.
Why do you care if he's your enemy? That's the least of your concerns, it's the fact that she slept with someone else what should bother you and make you break up with her. Cut that bitch out of your life.