Tell them this is on your top ten list of things that definitely really happened.

It'll probably be the end of the conversation. You'll annoy them, they'll know you're probably right, and since there's no evidence to discuss they don't have any material for prolonging the argument. Unfortunately, they'll still completely believe the next stupid thing they see.

Or...you don't want to have a fight with an elderly relative that's gonna die soon. In that case, I would just pretend you never saw the message.