Can a presumed father fight for custody and have any chance of winning. He has raised this child for 7 years.?

My husband was married to a woman who cheated on him. She got pregnant with a child that turned out not to be his. She told him the entire pregnancy that he was the father, when the baby was born they were married so my husband's name was put on the birth certificate. The mother was very wrapped up in drugs and running around so my husband raised the child along with her brother who does belong to my husband. When the kids were 2 and 3 my husband took them and left his wife, moving to a different state. About 6 months later the mother said she was clean and wanted to share custody of her children. My husband let them go with her, she had them for about 5 months. Then one day she called us and said she was hooked on drugs and could not take care of them, so we went and got them. This was four years ago, during this time my husband was granted a divorce and full custody of the kids. She never showed up to court to fight the divorce or custody so it was automatically granted to my husband. She has since spent the last 2 1/2 years in federal prison, now she is out and wanting custody of the children. My husband has raised this child her entire life both emotionally, physically, and financially with no help from the mother. However he is not biologically her father, if she takes us to court for custody what are my husbands chances of keeping his children? I know the children's best interest is to stay in their home with the only stable parent they have ever had. However she seems to think because he is not biologically the father that the court will automatically give her custody.