They have been divorced for over a decade.
His enslavement is to the state, paying for a child not his.
I find it incredible that any right thinking person could find this situation acceptable.
You ought to be ashamed.
If he weren’t married and had been paying child support (I’m against this btw) then there would be an injustice. Because he married her, there isn’t.
Has he been wronged by her infidelity, yes.
Is that cause for divorce? yes.
Does that annul the marriage? no.
I find it too common that people think marriage is about a guy having exclusive sex. Marriage is a societal institution set up to make it hard for a man to leave a woman with children. Marriage is about children and the needs of children.