my bff got cheated on?

so i'm dating this guy. my best friend is dating his best friend, so we're all friends and connected in a way. yesterday my boyfriend told me that my best friend's boyfriend cheated on her multiple times with the same girl, and i dont know what to do. i promised him i wouldnt tell anyone. but i really feel like i need to. he thinks it needs to come from her bf, but i'm not so sure it will. is it horrible of me to wait a few days to see if he will tell her? or should i just do it? it sucks because the guy's going to be so mad at my boyfriend for telling me and i dont wanna cause problems between anyone.. what do i do :(