What can I do if my husband is getting sued by the state for child support from a child that he only found out in 2014?

My husband found out through Facebook in 2014 that he had a daughter. She is fifteen and child protective services had her in custody because her mother abandoned her and her three brothers. I did a DNA test for her which came back positive. We never did and official test from the state. I just bought some test of the internet. From that time, the state has had her. She's a trouble child and needed to go to therapy. Never once did the social worker tell us that my husband needed to be paying child support or that he was going to be billed two years later for it. The child was sent to us 6/30/16 for temporary assessment for 6 months to see if it would work out with us. One month later, she ran away to her mother’s house and hasn't been with us since. The social worker knows all of this. We reported her as a runaway to the police. Not once after I emailed her the social worker has she called to see if she's okay or if she return but today at 5:30 am, we got a knock on the door. It's a sheriff officer saying he's being sued for impaired child support by the state.