MTL, if her grandmother cannot handle the truth, then cosmowanderer has already made a mistake in telling her grandmother that she is an atheist. If her grandmother is made of porcelain and will break, then keeping her atheism to herself would have been best. So, assuming that it was not a mistake to tell her grandmother that she is an atheist, I would (if I were in her situation) talk to her whenever she brings up the topic. And if my grandmother could not handle such things, I never would have told her in the first place. Doing this sort of thing halfway is generally a recipe for a bad outcome, a constant source of vexation for all concerned.
"A wise man ... proportions his belief to the evidence."
— David Hume, An Enquiry Concerning Human Understanding, Section X, Part I.
