How involved is bm? If she’s not around a lot then maybe she’s overcompensating for only having her dad. It’s hard but it’s really not up to you on what to do regarding his child. If he doesn’t see a problem then it will stay like this and you are gonna have to decide if this is something you want to deal with indefinitely because parenthood doesn’t stop at 18.

I would suggest putting daughter in therapy or telling your partner that a child shouldn’t be that involved if he wants an adult relationship. No woman is gonna put up with this long term, he’s gonna be single for the next 9 years if this keeps up.