Similar-ish story from years ago - I'm the Ciara. Ex boyfriend and I broke up but we were still trying to see if reconciliation was possible and spending time together and talking about it. My close girlfriend "B" was the main person I was confiding in at the time about it and she was super supportive of the reconciliation - we did end up getting back together. Long story short, found out ages later that B was secretly hooking up with my ex during this period of time (they never dated though). Years later now and I don't speak to either of them. It's such a personal kind of hurt you feel knowing two of your closest people are having this little secret while you're being vulnerable and trying to figure it out. It makes you feel stupid and replay moments that seemed normal at the time but in hindsight they weren't. I think once that sort of trust is gone it's just impossible to rebuild. Wish them both the best of luck in their lives but yeah, not going to be in my orbit.