mental illness is a serious issue if someone's obsession with conspiracy theories goes that far, and shouldn't be taken lightly
some people are just stupid though, and also they can actually NEED something like this in their life to focus on and be obsessed with. It can give them the feeling that there is some kind of bigger deal going on in the world, outside of their own relatively meaningless existence. religion fills the same kind of hole for a lot of people. it's not necessarily a bad thing.
if it's harmless just ignore it. I have a couple of friends back home who have gone a bit obsessed with silly conspiracy theories, and I just ignore them or gently take the piss.
because there are so many TRUE shady things that the various governments and powerful corporations of the world do, it can be hard for people who lack critical thinking faculties to identify the really wild and silly conspiracy theories, and they'll just go ahead and believe anything.