But, there's nothing you can do about what she wants to say.
I'm thinking an 18 year old boy probably isn't thinking of a 26 year old OW as his mom. I rather expect that if he hears it put that way, he might even correct her, or at least shake his head inside. At his age I don't expect that he sees any need for another mother figure. Try not to worry about it.
Anyway, don't say a thing, nothing good can come of it.
