If your friend is trying to "cure" the denier, then it might be helpful for them to learn some of the tactics for reaching/deprogramming cult members. One that might be helpful is "street epistemology".
I also included a thought experiment for him:
Imagine you take a conspiracy influencer and actually put them in the spacecraft with the astronauts so they could see the whole thing with their own eyes and wouldn't be able to deny it. They return to earth to tell their followers that it's all real. Do you think everyone will be convinced, or would they say he's now in on the conspiracy and find a new person to lead the conspiracy theory?
(I wonder if there is a sociological shorthand term for this?)
https://en.wikipedia.org/wiki/The_Final_Experiment_(expediti...
"The participating flat Earthers all admitted that the midnight sun was a real phenomenon. The larger flat Earth community has largely rejected the results and accused the participants, including the flat Earthers, of having faked the expedition and of being part of a larger conspiracy to promote the spherical Earth model."
When I was doing my student teaching, one of the teachers in my department was a creationist, but he didn’t seem to have read Genesis 1 at all because when I asked him about the firmament in the heavens separating the waters above from the waters below, he had no idea what I was talking about. At least the flat-earthers know enough scripture to follow their dogma all the way to its absurd endpoint.
Notes:
0 - https://rationalwiki.org/wiki/Flat_Earth
1 - https://rationalwiki.org/wiki/Crank_magnetism
2 - This is why religions such as Jehovah's Witnesses require members to proselytize (including going door-to-door) because outsiders are so adverse to the members that the other insiders remark things like "those outsiders are so depraved, that's why you can only be with 'true believers' like us".
This is one of the primary techniques of SE. People are open to answering questions and with some skill with Socratic questioning, you can get results. Dumping data just causes cult/hoax members to feel threatened because you're attacking some core beliefs and they will try very hard to avoid losing a sense of self value[0].
Notes:
0 - "What I believe is wrong" -> "I am wrong" -> "I am a bad person".