The tricky thing... you can't let it feel like two against one, like you and your bf have already decided on this plan and now she's forced to go along. You'd need to talk to each of them about it separately first and really get them on board.
If they do want to stay together, this may be the thing that spurs them to work on a healthier relationship. And if they don't, then it just has to end before things get more toxic.
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.