Cheating friendssss? ?

So I have a bestfriend lets say her name is Sarah, shes dating a guy named lets say Ryan (were 13 btw) so a big group of us get together to go bowling, and everyones pressuring sarah to make out with her ex boyfriend (who still likes her) and sarah says she doesnt like Ryan anymore and she is goin to dump him but before she does, sarah & her ex boyfriend make out. Sarah dumped him later that night. Does Ryan have a right to know what happened? I mean a couple weeks before ryan made out with a girl on the bus while he was dating another one of my really close friends. My point is if ryan ever found out id feel bad because i know what happened and didnt tell him. what do I do?