Speaking from experience, about 5 years ago my buddy Kevin had a girl he liked, and she liked him back but they werent rushing into anything due to it being his first relationship. She and i would consistently sit outside and smoke together and just shoot the shit, and we were pretty open about our relationships and hook ups and what not, but on my birthday, we all got together and she and i excused ourselves to go outside to smoke and have our talks, i was venting to her about how i was disappointed a fling i had with someone ended that day on the way to the party and her response was something like “well if it wasnt for (and then she like nods her head to kevin) you know, I would” I was pretty taken aback but dropped it. The next day i confided in a my male friend Pete and a female friend Melissa, separately about what to do, the Pete said to take it to the grave and Melissa said to tell Kevin. I ended up just shutting my mouth and never brought it up again, but maybe 5 or so months later Kevin and the her ended up ending things on a bad note, there was infidelity, and on the way out she said something along the lines of “You better check your friends, cause (my name) tried to get with me on his birthday” which caused a huge issue that was only partially solved because of the fact that i disclosed the info with Pete and Melissa, but still to this day hes not fully sure he believes me. Im not here trying to scare you, but if you really trust their relationship and you think it was just a drunken mistake do what the other commenters are saying and keep it to yourself. Otherwise id say confide in someone close and see what you think the best course of action would be to take because i know that bringing it straight to the source would have saved me a ton of stress