Is my husband liable for past child support?

My husband found out Thursday he is most likely dad to a 12 year old girl. He had an affair with her mother while her husband was in prison. My husband suspected she could be his over the years but wasn't sure. Her and her husband remained married for years after the child's birth and in the last couple of years they divorced. Now the mother has told my husband he is the biological father. He wants to have a DNA test to be sure of paternity. My question is, if DNA is confirmed he is father could she go after him for past child support? By what I am reading she can't because her ex-husband is legal father. The state the child was born in was Mississippi. She now lives in Louisiana and we are in Oklahoma.