This isn't possible. One gives birth, the other is merely a spectator and had nothing to do with conceiving a child.
This kind of thing really hurts the writer's overall credibility, especially when he argues at another time that a man who once lived with a child's mother should not owe support for the child when the adults separate, even if the child considers him a parentlike figure.
She is not the child's mother. She is a woman who once lived with the child's mother.