What to do?

My best friend in the whole worlds birthday was this past Saturday! So a couple of our friends and us had a Bon fire. We were drinking, smoking weed, you know what kids do. And one of our friends says he likes her, but he never talks to here unless it's about sex. So around 2am everyone went inside and our friend who likes my best friend had sex! I would have never thought of this to happen. I don't remember much but Monday AF  came in for a visit. Me and her are both talking to other guys. Should I tell her!!!!! Should I tell Luke!???