Kids custody problem... asking for my boyfriend?

I know my boyfriend is under big stress these days but he is carrying it on all by himself, and Im just sitting here supporting him silently. I saw him searching for kids custody online and I know he is really trying his best to figure out something.
So the story is here...
My boyfriend literally have 3 kids. His ex wife had a daughter with her ex, they went apart when that daughter was 3 months old, and she then met my boyfriend, they got married very soon and my boyfriend literally raised her daughter, sooner they have their own babies, one girl one boy. So right now, the oldest girl is 10 years old. Her real dad, who is his ex wife's ex came back to get the custody back. I know the girl is not my boyfriend's daughter biologically, however he did raised her, supported her financially while her real dad did nothing at all. However my boyfriend can do nothing to it since biologically he is not her dad, so right now the girl goes to her real dad's place one weekend then my boyfriend's place one weekend, and she said she hates it, she doesn't like her real dad since she only willing to call my boyfriend daddy.
I couldnt imagine how bad my boyfriend feels since he treated her as his own daughter for 10 years since she was a baby.
I dont know where can I put this question up but anyways.. if there is anyone knows what can we do to figure out a way? Or if there's a way to fight for the custody? Sorry if I sounds ignorant on this field since I have no experience but I just cannot bear seeing my boyfriend suffer from the pain and I really want to try to help and dont want to break his ego.
Thank you so much for reading...
Kids custody problem... asking for my boyfriend?
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