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".

Link: https://en.wikiversity.org/wiki/Street_Epistemology

That's a bit too much to ask of these folks, I'm afraid, but I did boil it down for him by saying that the real question is understanding why he is inclined to believe its a hoax, rather than trying to pile up evidence in front of him. I suppose that's what "street epistemology" is, though. Nice to have shorthand terms for these ideas.

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?)