[Total Light Storm]

Hey, welcome to TH,

First of all, none of this is your fault. It's her dad's fault and her mom's fault for treating her that way. You are just her friend, probably the same age she is, there isn't a whole lot you can do in a situation like this except try to be there for her, which you have done, tell the authorities, which you have done and just know that you aren't responssible to make decisions for your friends safety, that's the job of adults, such as the police and other adults. You promised you would keep her safe, you tried your best to do that for as long as you could, you aren't able to watch her all the time because you both live at different places, which means none of this is your fault. The police should have listened to you and your parents when you told them what had happened.
Do you definitely know that she went back home to her parents? The police could have put her in a foster home and not told you about it.
I think you should try calling her house, see if she is there and if her parents will let you talk to her, if they answer and tell you she isn't there, ask them where she is.

I hope this helps, if you ever need anything, you can PM me.