OMG! Messing with someone's man😳

Ok sooooo I love my bestfriend. We lived in a dorm together when we were freshmen anddddd we're really close and we're always honest with one another whether we agree or not. So she's been cheated on plenty of times and she usually tells me about the guy she's dealing with except this time. I saw her post a picture of this guy on snapchat and I saw his face and realized that's one of my friends' boyfriend. Asked my bestfriend about it, she says that's her boyfriend but he already has a girlfriend and I told my bestfriend but she doesn't believe me. I also told my friend and she doesn't believe me either. What in the hell should I do now?