What does one say?

[Lethe]

She got hurt when I made the comparison, started ranting, saying things like, "I hate it when you say the one thing that kept me from killing myself is a lie." and "God is real. Santa isn't."

There are multiple routes one could go with this, though at the moment I can only think of two. You could distract her with an ill-conceived joke or meme: "The cake is a lie too," or you could go all C.S. Lewis on her: "A man (or woman I'm assuming) can no more diminish God's glory by refusing to worship Him than a lunatic can put out the sun by scribbling the word, 'darkness' on the walls of his cell." If she is secure in her beliefs and they provide her with a genuine measure of comfort, your lack of belief should not upset her. Attempting to dissuade her of her faith would likely do more harm than good. I'd wager it be best to reason with her through her faith, as it's obviously a delicate issue for her.