If someone believes deeply that they can fly if they jump off a cliff and flap their arms, is it putting them down to tell them it's fiction and things will end badly if they try?

I hardly see any mortal danger here in a way you do. If you really want to help try making your comments sound less aggressive. Now it seems as if you are jumping out of a bush and beating people up with a club because you think they are headed for the cliff. You must have already noticed what are you trying to say is rarely taken at a face value because of that.