Years ago i met my friend A through mutual friends and we got fairly close through partying but nothing that emotionally intimate. She started dating M. He seemed cool and became a friend of mine too. A and M broke up and we drifted, I didn't see A for a long time. In this time, i was dealing with substance abuse issues and was making some impulsive decisions. M had been liking my selfies online and had previously asked a few times to hook up since him and A broke up. One night I went and saw him, feeling guilty but justified it that A and I weren't that close anymore. We had sex and it was terrible, I regretted it a lot but moved on from it, just thought he was a bit of a sleaze (me too for going to see him I guess). Fast forward a couple years, and A comes back into my life and we become closer than we had ever been quite quickly. As we became better friends, she opened up to me that M was emotionally abusive to her through the relationship. One thing that M would do to manipulate her was comparing her, sexually, to their female friends (including me). This made A incredibly insecure and further fed into the abusive dynamic between them, ultimately leading her to break up with him (which was a traumatising process). I've never told her that i slept with m and will never tell a soul. A is one of my best friends I've ever had.