But with the caution that "the conspiracy would have to be unmanageably huge and would eventually leak" has not been proven to be hugely effective in changing minds of those already committed to belief -- if it were, we'd see more impact among flat Earthers, Apollo hoaxers, and the like. It may be a more effective argument to make to those on the fence, or as an "inoculation" against falling for woo that somebody will encounter in the future.
Adapting advice from "Pastels and Pedophiles," a book about the Q-Anon conspiracy theory complex (to which the anti-immunity movement in response to Covid prevention is closely tied), arguing with a friend whose fallen into the rabbit hole is as likely to be counterproductive. Might even create confirmation of the conspiracy, a la "See, even my friend has been recruited to spread the Official Line to me!" The approach they recommend in the book is to keep communication open, acknowledge and validate the emotional stresses that make the conspiracy attractive ("Yes, I understand that you feel concerned about this stuff, it's a scary situation!") and being somebody outside the conspiracy echo chamber to whom they can maintain contact. They also advocate a gentler "Socratic Method" (my term, they do not use it that I recall) of "That is interesting, can you share how you KNOW that" to hopefully lead the victim of the hoax to asking their own questions about the folks who are feeding them nonsense.
The hardest thing, for me, is to make peace with the fact that you (I) cannot rescue folks not wanting to be rescued. There is no magic wand to wave to make people see sense. You can be available as a friend who has not fallen into the Rabbit Hole, there to help and accept when/if they decide they're ready to climb back out.
But I'd note that I have a different approach to helping a friend who has gotten lost in nonsense than I would have dealing with somebody spreading nonsense in front of an audience to be recruited, such as on Internet forums, where the goal might not be to rescue the one who is lost (or is a troll, no way to know) but is instead to block them from fooling more people. I get a little more confrontational there!