Not mentioning feels a little bit like sneaking. Particularly if he notices at some point and she doesn't want to explain.
Cadogan will absolutely teach her the Thomas family home-sign on top of BSL. And Roisin is going to demand she speak Welsh when she comes to visit. Whenever Cadogan stops panicking enough to admit she should have dinner with him and his mother.

They just need to talk about it. That'll help.