Interesting. You sound like you are referring to a FinDomme having a son, who ends up growing up & becoming a finsub?
I feel the Mother would only get involved if the son's FinDomme was stepping over a line. I can't speak for any FinDomme of course, but I would say a genuine FinDomme would be happy that her son was in such a relationship, assuming it was a healthy Findom relationship.
A question that springs to mind is does the son know his Mom is a FinDomme?
If so, has the son told his FinDomme that his Mom is a FinDomme as well? All kinds of possibilities if Mom & son's FinDomme get talking!