"Radical candor" is a new term for me but about 11 years ago I encountered references to "radical honesty" which seems similar. I will look up "radical candor".

I think what Divine suggests is not radical candor, though, because she includes "I’d not tell her that her activity is boring for you or that you saw a better event. " and I do appreciate these pointers as they soften the message. So candor, yes, but maybe not quite as radical.