Where to begin...

My husband has a 10 yr old daughter. Her mother is very controlling. For example, she has a phone and if she doesn't reply to her mom while she is with us then she loses her junk and calls cussing and screaming at her. That's only a tiny part of the problem. This past April they went to a child support hearing and switched it to 50/50. Just child support not the court order paperwork stuff. Well fast forward to August. Bio mom says she dont like how things are going cause she wants her to follow her rules even while she's at our house. So when school starts we were switching it back. I was under the impression that the joint custody was through the court not just child support so I told SD that she can't do that. Maybe I shouldn't have but SD was crying and I was just trying to reassure her. Well bio mom flips of course saying its whatever she says it is. Well we dont see SD for 4 months. She comes back in Dec first weekend was weird she was acting reserved. Well her next weekend comes around which she comes on Saturday cause her stepdads birthday. My husband allowed her to come Sat instead of Fri. Well on Sat her mom text and ask her daughter if she wanted to go to a Christmas get together at her families place. My husband said no cause it was his weekend and she had already come late. Well about an hour later SD walks right out the door. Husband is calling her name she ignores him and walks out and says mommy is here and closes the door runs to the car and gets in and they drive off. So husband text bio mom and says he said no to bring her back. She was supposed to stay til Christmas eve at 6. Well bio mom says she didn't know that husband said no but SD can make her own decisions if she wants to go she can. Husband was livid and calls the sheriff's office and one comes down. Well turnabout he was her cousin. Husband hands him the visitation agreement and he says it ain't signed by a judge so their ain't nothing he can do. He can't force her to come back. She doesn't come back. Hasn't been back. She came to the house and got her meds that she left cause she was sick and husband told bio mom a  SD that what happened that day wasn't right and bio mom said SD is going thru things. But that dont give her the right to walk out the house without a word and leave with mom or not. He told he he was done and can't do this no more. If it ain't one thing its another and hes tired of the fighting. Bio mom said once again she didn't know but why did she allow it? She made it seem like it was ok for her to just walk out. It upset me to see my husband upset when all SD had to do was listen to her father. But I feel like Bio mom said for her to leave cause if SD doesn't listen to her mother she literally loses it on her. She has to control every aspect of her life to the point of havi g a doc diagnose her with ADHD when we think she doesn't have it. She dont act like her mother says she does at her house. What should we do? He dont want to give up on her but hes tired of fighting. Then the paperwork he has is pretty much useless. Hes frustrated. He wants joint custody again. But he feels like hes fighting a losing battle. Bio mom has a case open on her with DSS. For domestic violence and abuse. We know of the fighting and SD has said stuff about bio mom being mean and hitting for no reason. But SD is protecting her mom tho. Wont tell them the truth. Like she is terrified of her mom. Idk what I'm asking but some advice or anything would be great. Thank in advance.