Is your bf addressing this issue at all? Like, has he said to her "Please don't disrespect my fiance's wishes and invade our time together like that" or is it all on you to try to resolve the problem? Does he see it as a problem?
Also, has he, to your knowledge, talked to her about why he's not ever sleeping in her room any more? Her way of handling it is inappropriate but I'd be feeling pretty freaked out too in her situation and if he's just allowing this situation to devolve into toxicity, that's not ok.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.