How can i deal with my 15 yr old's move to Dad's (who manipulated her) who is her friend not parent?

I feel so sad and betrayed, mostly heartbroken-my daughter who 'hates' me now and blames me for all her problems/failures..she was diagnosed with depression last year..I've taken her to every referral we received for help, and as her depress seems better (she's on medication) she's an emotional timebomb, and is failing school (is very bright, doesnt want to do ANY work). After attending family therapy, her father renegged on his promise to the social wker he would back me up on issues (consequences for behavior) and agreed he wouldn't entice her to move in with him, as such a change esp in schools could be damaging and manipulated her into thinking the prob is 'her environment' and she should move in with him (where there's no structure or supervision )..he's a buddy not parent,and treats her like she can do no wrong..She became violent towards me after I refused to let her move, and it was recommended I let her go. In two weeks she hasnt even called me, and stopped taking her meds.