I would want to know, yes. I think I would do my best to understand the thinkng if something so big was hidden from me, and try to be sympathetic, but I also think I'd be hurt that I wasn't told, like I wasn't trusted, and like I wasn't being given all the relevant info a situation that intimately involves me. You don't need to tell her the details, or vent, but why can't you just tell her that there was a boundary violation?
The major players. Me, 30ish bi female. Gia, girlfriend of 4+ years. Clay, boyfriend/dom. Davis, ex/friend/"it's complicated." Eddie, roommate & fwb.
The supporting cast. Eric, Gia's husband. Bee, Gia and Eric's toddler. Dexter, Gia's lover. Helen, Eric's lover. Izzy and Nikki, Clay's partners. Liam, Eddie's husband.