Barb is a grown woman who worked for decades, is a widow of someone who worked for decades, and currently has access to pensions and/or social security. I don't see why it is Chris' responsibility to pay her dental bills. If it were, why is it not Cole's as well?
I think Chris has an obligation, if he is living with his mother, to pay approximately half of all joint expenditures. After that, the money is his. and he can do whatever dumbass things he wants with it. If he buys his mother some dental work, that's great. If he buys lego, I don't give a shit. If he buys lego and then bitches about teeth money, I don't give a shit.
It is a different story if he doesn't currently pay his share, and Barb is saying something that in effect means "You have to pay your share now. I can't cover you because I need dental work." But if he is contributing, then Barb's teeth are Barb's problem.