If I were you, I would say clearly to my partner that I was NOT comfortable with him being intimately involved with this woman and probably never would be. Too much dysfunction there, it'd be inviting drama into your lives. And yes, it's ok also to recognize that this is probably not a healthy situation for her and to not want to see that continue/worsen. Hell, I don't think I'd even be ok with the sleepovers... she needs a chance to move on...
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.