Best friend tried cheating on her boyfriend with me.

me 20M and my friend 20F have been friends for like a decade at this point, and over that decade the friendship has fluctuated. we even tried being romantic at one point- but she decided against that, so we just stayed friends. We’ve both had relationships since then while still being friends and it was never a problem. Anyways, I hadn’t seen her in person since she moved to the east coast a couple years back, but I finally got around to visiting. It’s important to note that recently, she got a boyfriend at her university. Anyways, I stayed with her and another person for a few days, which we had planned for awhile. The first day and a half was fine, but she started getting more and more flirty and touchy after that. That’s kind of just her personality, so I didn’t think much of it at first, but after a while it kept getting weirder. when it was just the two of us in the room she started talking about how horny she was and a bunch of other weird shit. I never reciprocated because I don’t want to be that guy, but she was pretty determined. she started asking questions like “do you think (xyz) counts as cheating?” to all of which I said yes, that is in fact cheating. Then, she incredibly bluntly said “Yeah, I don’t think me and my (boyfriend’s) relationship would end from cheating… we like each other so I’m sure he’d forgive me.” I had no idea how to respond to any of this, but I was able to awkwardly change the subject eventually. even after that, she tried holding my hand at different points, and just generally flirting in the most heavy handed way possible. I got the hell out of there without anything happening between the two of us, but now I don’t know what to do. It seems like the “morally right” thing would be to tell the boyfriend that she tried to sleep with me, but there’s also something to be said about standing by your friend even when they’ve done wrong. If one of your closest friends tried cheating on their partner, would you tell the partner, or stand by your friend? I should mention, the two of them are about to move in together, so if I were to tell the boyfriend and they split up it would have huge consequences for my friend. I don’t know the boyfriend at all, but he seems like a nice person- and he certainly doesn’t deserve to be cheated on. I’ve told three people about this situation and surprisingly, all three have said that it’s not my responsibility, I should just keep it to myself, and just distance myself from the friend at the most. That seems like a huge cop-out to me, but maybe not. Would you tell the BF?, talk to the friend first?, do nothing?, unfriend her- but tell no-one? I’m at a loss.