One question is: can you stop your friend believing these conspiracy theories? Regrettably, almost certainly not, at least not without a huge investment of time and patience. People are free to think whatever they want and some of us put that freedom to the weirdest uses. At least we can be thankful the conspiracies your friend has latched on to are about objects in the sky and not, say, which reptilian species is secretly controlling things.
A different question is: can you change the norms of the relationship so you don’t have to engage with this? Happily, that’s a different mission.
This seemed like good advice to post here because I know a lot of people are wanting to maintain friendships with such people (or family relationships for that matter) despite their nutty beliefs.
One way to counteract a conspiracy theorist is counterintuitively not to argue against them but instead simply ask “Ok, well fine, how will this theory make your (and/or our) life better in the here and now?” Keep the theory circling back to them and their life so as to get them to self-reflect as why they really want to believe in such a theory because that is the real issue that you should be addressing, not the theory itself. This method is better in face-to-face discussion and needs a lot of patience.