So my girlfriend and I are talking the other day and she tells me "promise you won't tell anybody this, I wasn't supposed to tell anyone" (me and her will share 'secrets' all the time) so I'm ok what's up. One of her friends broke up with some dude a few months ago and started dating one of my friends shortly after, well a few nights ago while my buddy was gone for the holidays she went over to her ex's house and stayed there until 2 in the morning... she claims all they did was watch movies but she's kind of a filthy slut (plus she's been caught before lying about stuff not happening with people when it definitely did) so I'm 90% convinced that something did happen. My girlfriend made me promise not to tell anyone, but damn, I really want to tell my buddy, you don't keep shit like that from people ya know? I tried reasoning with the gf but she says it's not our issue, and she claims nothing happened so why cause the shitstorm for ourselves for no reason, and claims this isn't a big deal. I said that if I were in his shoes, I would really like to know. What do I do here? She really doesn't want to get pulled into their fight (my gf was the only one that knows about this, so if it gets out the girl will know who it came from, and it'll affect their relationship) but my friend is such a good guy and I really think he should know. How do I deal with this? If I tell, I lose my gf's trust. If I don't, I'll feel like a dick.