Wow, her boss's wife? That would change things for me. I would call her out.
"You know, you ask me about this every year. Let's go talk to DH since it seems to be something that bothers you."
And then I would bring my husband to her and ask her to repeat her questions "in hopes that we can get to the bottom of why this is bothering you so much."
Your mom doesn't have to endure that from her husband's employee. Once or twice I might let it pass, but year after year? No way. She needs to stop.
ETA: And I say this as the wife of a man who supervises a lot of people. I wouldn't put up with that from one of his employees for a second.