In Network charge dispute

My daughter lives in South Carolina. She had to take her child to Urgent Care for an ear problem. When she arrived, she was asked for her insurace card and she made sure she ask the clerk if they were in Network. She said they were and proceeded to ask for her co-pay. She paid, was taken care of and left. A few months later she had to return with another ailment with her child. Another copay and treatment and child was take care of. A few months later she receives a bill from the doctor's office for $600. She calls her insurance company and they inform her that this office was not in Network. She then calls the doctor's office and they tell her that they just became in Network and was not when she visited. Neither the office or the insurance company will accept responsibility.  Why did they tell her they were in Network and why would she have to pay this bill? Who can she contact?