My eldest used to live with his dad and well let's just say that it was with less than desirable behaviour. He has moved back with us and like you said he is no angel, but nothing like what his dad was saying either. He was apparently having constant run ins with him and his step mum (although that is a whole other issue with her treating him differently to her biological son).... which don't tend to happen here with us too much.

I tried many times to talk to his dad about it and how he felt about things there etc but was always met with excuses, reasons and throw the hands in the air it's all too hard... now this is my ex, not my ex's wife/gf etc. I'm sorry but I really don't think that she's going to listen to you anymore than my ex listened to me. I think that she will probably tell you where you can get off and that she's not your daughter etc etc etc. I personally would probably not go down that path. Keep your communication line open with your DSD though because in time she will remember what you did for her. I know I've probably not really helped you much sorry.