Step sons mom took his money

For the past couple months my step son has been asking to do stuff around the house for some extra money to get his girlfriend a gift. Her birthday is on Valentines Days so he wanted to get her something really nice. He saved up about $400 is what he told us and he kept it in a plastic. When he came over for his dads time with him he was very pissed. His mom had went into his room and took all his mom. My husband called his ex and they got into a fight. She said she used it to pay bills. Well honey your son is 16. It's not his job to pay your bills. Especially when you stole from him. She also has no intention of paying him back because she keeps a roof over his head. That is such a bullshit excuse to steal from your child. He then tells me his little sister*they have the same mother, different father* said she saw their mom go into his room while he wasn't home and after she came out their mom said she was going to the Casino. So she pretty much lied and her daughter ratted her out. Also how the hell do you blow $400 at the casino?! I give up after losing 25. I told my step son to just start keeping his money at our house. I talk to my husband about taking $150 out of our savings account to give to him so he can do something nice for his girlfriend. I know its not his full $400 but at least it's a little something.