AITAH i want to tell someone his gf cheated but my friend says no

I(F20) was with this guy who had a girlfriend and i know 100% it was wrong but technically nothing ever happened. we started as friends all hanging out in a group together me and my best friend(F20) and him and his best friend (both M20s) eventually we all became a little too close to where we were having sleep overs and at the end of the night we would split up into pairs and go sleep in our beds together me and him and them. When they would go to their room stuff would go down but with us we kept it PG never even cuddled or anything. Not that it was right because we knew feelings were involved and she didn't know and my best friends BF didn't know either. Eventually we ended things between me and him because we knew it was wrong and i told him he needed to tell her and decide if he wanted to stay since i think it isn't fair to stay when you know you have feelings for someone else. My best friend and her guy ended short after and she went back to her boyfriend. None of us ever told the perspective partners. I believe that someone should reach out to his girlfriend and let her know that something happened, but my best friend doesn't want me to because she's worried that they will tell her partner. We almost had a slip up once and the guy i was with decided to not tell anyone including my best friends partner he recognized that we all put ourselves in the situation and he didn't want to try and get even by telling her partner. Now me and my guy stopped talking but her and her guy are still talking. I want to tell my guys girlfriend about everything between me and him because i think she has every right to know but again my friend said no. Am i the a**hole for wanting to tell her even if i make sure my best friend isn't involved? please give brutally honest answers