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The original was posted on /r/aitah by /u/Salt_Draw2013 on 2023-08-11 08:12:54.

My best friend (42f) and her partner (38m) have been together for a decade. Living together and traveling around and have the same job industry. I’ve been their very good mutual friend for 7 years. He wants to propose to her this winter. However I’m against this because he doesn’t know she had a 5 month affair with her ski instructor all winter while on a work assignment. She lied and cheated and called and told me all the graphic details about meeting in abandoned ski cabins to hook up, even lied about staying at my house when she was really with her lover even when I asked her to leave me out of it, I don’t approve or want details. I’m good friends with both of these people and we have travelled together often and hung out maybe 2-4 times a week in all kinds of adventures like rafting and hiking etc. for many years. she refuses to tell her boyfriend and said he’d break up with her if she did. I just can’t stand it and goes against my moral code. It wasn’t a one time drunken thing. The only reason she ended the affair is because the ski instructor had more than just one ‘special’ friend and she was upset about it so decided to stay with her boyfriend even though she had called to tell me she was falling in love and would end her long term relationship for the hottie ski dude. I decided that if they knew my partner was cheating and didn’t tell me, I’d never forgive them. So I took her boyfriend out to lunch and told him. He didn’t even have a hard time believing it bc he never liked her energy with the ski instructor. But now she has turned our other mutual friends against me and they think I should have stayed out of it. Some of them literally told me it’s what happens in a ski town. But her boyfriend and her were not in an open relationship and he certainly wouldn’t want that. I just figured he should know before getting married and decide if that’s what he wants with all of the facts. I did feel bad because his brother just died, but felt the truth couldn’t wait any longer. This was not her first affair and wouldn’t be her last. I almost just stayed out of it but she told me she didn’t use protection and I just lost it. Potentially bringing home an std is just so additionally disrespectful. I have lost all respect for her as a friend at that point. Anyway AITAH for revealing her affair? Should I have just stayed out of their relationship?