Several years ago I was offered free counselling through my workplace. I only attended one session, and in retrospect I have nothing but sympathy for that poor woman. I don't recall everything we talked about, but I do remember at one point trying to explain Dennett's Benign User Illusion.

"How would you help someone suffering from an existential crisis" seems as much a valid question as the top-level post question, into which you'd think the LW community would have some insight. I'd imagine people suffering from existential crises are great candidates for introducing to rationality, but beyond "so, you've decided there's no God..." I'm not sure they generalise all that usefully.