It angers me to see my best guy friend sad because of her, help?

Okay, he is my best guy friend and his girl really screwed him over. They are broken up now but he is still suffering, all the while she is enjoying her life without thinking how bad she hurts him. I used to be her friend before but I'm not anymore simple fact that she kept being a BLEEPING BLEEP. Any who, this isn't the first time she cheats on him, and she still has the nerve to say she loves him. Is it wrong that I'm angry and saddened by his suffering? I mean yeah, it's natural to be upset that my friends are hurting, but is it bad that I cry because of it, in this situation? maybe i'm just making something out of nothing