Il willing to bet, if that day comes, she just might call you up and yell “You never told me he was dating a trans woman! How dare you keep this from me!” And then blame you. By the way she just flippantly changed the conversation, I wonder if she’ll even remember.
Or she could take the superior route. She might try to impress her friends with how she is way more loving and Christian than they are by accepting her grandson and his girlfriend. Perhaps if she talks down to them and brags about how it makes her better…Maybe she’ll preach at them. Then again, that may not be how the Catholic Church works… fascinating story!