I used to be a 9/11 Truther 😭 Turned out that I, too, was mentally unwell and socially isolated even worse than that hairy fella on the island with a face painted on a football. It's crazy (if I can use that word in this context) how your brain can look at something and be 100% convinced of it, and the exact same information can look ludicrously and obviously bullshit a year later when your psychological well-being is in a better place.

Advice: don't bother debating conspiracists on their 'evidence', but when they hopefully emerge from their pit of psychosis, welcome them back with open arms. And, perhaps most importantly, ensure they know that that non-judgemental welcome is there for them whenever they want it. The less social friction/cringe/embarrassment they feel is awaiting them, the easier and quicker they can transition into some sort of normality again. Debating the details of whatever conspiracist horseshit it happens to be is like trying to cure schizophrenia by proving the person's hallucinated demons wrong with FACTS and LOGIC. You'll just make it worse.