Having him call her “Mommy” anything is obviously really messed up, but to me, teaching him to refer to you by your first name is just as bad and worthy of NC.

We caught our 2yo trying out my first name and her dad’s first name. Of course, she’s just curious because she occasionally hears us saying each other’s first names, but we shut it down very quickly. Not harshly, but also not with a smile or giggle, because that can absolutely encourage her to keep doing it (she loves to get us to laugh by being silly).

As a teacher, I find it incredibly disrespectful for a child to call an adult by their first name unless it’s been invited by the adult (for instance, being called “Aunt Sara” is fine if that’s what is taught by the parent and invited by the adult). Generally, using first names for parents, teachers, and grandparents is not a normal thing, and even kids know that. In the rare event that a child refers to their parents by first names, it’s a huge raised eyebrow for teachers. Specifically, concern over boundaries or lack thereof at home. Those boundaries with parents shouldn’t be blurred, and they certainly shouldn’t be blurred by a third party.

I’m tempted to tell you to insist that he never use the “grandma” title again (because she doesn’t deserve it), but I truly feel like teaching a child to flirt with those respect boundaries is dangerous for the child.

I think that investigating your legal options to cut her out of his life is the way to go.