Hi everyone!

I haven't posted on this particular group before, but have been a member for quite some time.
This is about support for myself but involves my son. About 2 months ago his uBPD girlfriend broke up with him. She admitted to cheating while they have been together, which was 5 years, as well as being involved with a man at the present time.
She brought into this relationship her daughter, (not my son's) who was only 4 months old at the time. The first year, she did not present herself as she eventually became. As time progressed, she wanted to go out clubbing more and more, had men in her life and even went on trips with them telling my son they were "friends" and even has become a nude model on the Internet. In the meantime, my son was at home with the kids - they had our grandchild 3 years ago together. He has treated her now 5 year old daughter as his own all of this time. He was her primary caregiver, especially for the last 3-1/2 to 4 years since she was absent from the home so much. He took her and picked her up from daycare/school, took care of the house, financially supported everyone, and was just an all out good dad to both of the kids showing no partiality at all between the two. Her daughter and my son have definitely formed a bond as he was really the only stable person in her life, the only one she could count on to consistently be there.
Now since the split-up, her biological father, (who has been in the picture every other weekend) and the ex-uBPD seem to be slowly weeding their daughter out of my son's life. Okay, when looking at it objectively, it makes sense her biological father would want her more since the breakup. He told my son he is so happy he had my son and didn't have to worry about his daughter, but now he is afraid she will have all sorts of men coming and going and is afraid for his daughter.
Through the 5 year old, it came out to my son and to us that she was seeing a therapist. They never told my son anything about this, but are still dropping her off on Sundays to spend the evening and night and he takes her to daycare Monday morning.
I don't know what is going on and neither does he. She had a T-ball game last Saturday and asked him why he wasn't there. He told her that no one told him about it. So it looks like he is being ousted out of her life and no one is saying anything.
As a mom, I hurt for him every time I look at the sadness in his eyes. It's almost not having the daughter in his life is effecting him more than losing the ex-uBPD.
Does this situation make sense to anyone? Her biological mother/father (definitely father, not sure about mother) has been taking her the counseling and they haven't even told my son about it. At this point, they are leaving him out of everything except when they want to drop her off and need someone to watch her. Him, being who he is, is not saying anything because he wants to spend time with her. I just don't get it. Maybe I'm not seeing the forest for the trees.
Anybody have any ideas?