Cheating

My boyfriends friend cheated on his girlfriend, or at least we were told he did at a party me and my boyfriend weren’t there. He’s cheated on past girlfriends and I’ve never said anything. I felt it wasn’t my place to, but I have decided to tell her that there’s a possibility that he may have cheated on her because i can’t stand by while he hurts another girl. My boyfriends supports me telling her he encourages me to do, he said he would do it but it’s probably best if it didn’t come from him. I know it’s probably gonna ruin his friends opinions of me but I really don’t care because I’m not gonna watch him ruin this girl. I needed to rant and I’m hoping I don’t chicken out. I know if it was me I’d want someone to tell me