Is revenge ever justified? Please help me decide what to do in this very unique situation..?

no dude, your best bet is to move on. you had a good relationship with her for about 5 years, so just let the last year stuff go. trust me you will find someone better who you can rely on, so don't waste any more time getting into fights with her or the dumb bf. In the future, she will probably cheat on him again and get caught, so you don't need to interfere.