Search Results: seeing my ex and his new girl

So I made a new group of friends because we all joined this new organization on campus. Out of everyone, there was one guy in particular who I got pretty close to, so we'd hang out alone a lot. He's flirted, but we've never kissed. Only cuddled on the couch when watching TV. But I just found out that he actually has something going on with another girl in our group.

So I'm not sure how to approach the situation. I've since backed off because I don't want to be the other girl. And I don't want to be involved with someone who's not even trustworthy. She mentioned to someone else that she's really falling for him. We all met each other at the same time, and she's a really nice person. I feel torn because I feel like she SHOULD know that he's not faithful (if he's hitting on me when we're all in the same circle, I'm SURE he's hitting on other girls, too).

I'm choosing not to say anything, but I'm feeling guilty because if I were her, I'd want to know. I've been cheated on before by someone I really cared about, and I was devastated for a really long time. And it bothered me that my ex's roommate's knew he was seeing someone else (even though I understand about loyalty, it still made be feel dumb). Am I right to keep quiet? I feel like I don't know the status of their relationship anyway.