This is kind of confusing, so bare with me.

My SO of 3 years has a 3 year old daughter out of a friends with benefits situation. BM is totally nuts; SO was the best dad ever and she was so grateful for him up until SD was 6 months old and SO told BM we were together. All of a sudden, SO became just a sperm donor and BM's first babydad became SD's "daddy" because SO "abandoned her." It has been almost 2 years now since SO has seen SD; domestic relations has screwed him over and was not able to file for any custody or visitation and he stupidly didn't take action right away like he should have after BM cut off all contact with him and changed phone numbers/addresses/etc.

So for the past 2+ years, SD3 has been told that BMs first babydaddy is her daddy. She is aware that SO exists, but is told he doesn't want her (although he gets SD5 from a different BM regularly and is very involved in her life and she is aware of that). She went to first babydaddy's house every other weekend there for awhile, but now BM says this guy isn't stable enough to have her kids overnight so he just gets them a couple hours every other saturday. SD3 obviously has a fatherly relationship with this man.

Custody wheels are finally turning and SO is trying to get 50/50 custody of SD3. There is no reason he shouldn't be able to obtain joint custody; BM is very toxic and unstable and there are page and pages of evidence to back that up. My question is this: what is SO to do about SD's relationship with this man she thinks is her daddy? Is he supposed to just rip her out of his life and pretend he never did anything to help raise her when he wasn't able to? SO doesn't particularly like this man, and he really isn't the best father figure or role model for any type of child, but that doesn't change the fact that he has a relationship with SD. What should SO do?