My friend cheated on her boyfriend, should i tell him?

Okay so, I'm living with a few roommates, and one of them happens to be one of my best friends. I've become pretty close with his girlfriend as well, who I discovered not long ago cheated on him with his best friend. She was drunk, and he was drunk. And. Yeah. She cheated. She doesn't know that I know, because our other roommate who she confided in spilled the beans to both me and my boyfriend. I immediately felt like someone just punched me in the stomach, because I'm so close to her boyfriend. I want to tell him, because if I was him I would want someone to tell me. But I don't want her upset with me, or the other roommate who she had told her secret to. But doesn't it make me worse of a person to keep this secret than to just let her boyfriend know? I know if I was living in a house full of people who knew my significant other screwed around behind my back, but never let me know, I'd be pretty humiliated and pissed off. What should I do?