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He later admitted that he made a mistake.
If you were in my shoe, what would you've told a person that says bitcoin is a ponzi scheme created to scam people?
That is what he possibly feels right now.
And If I'm also in your case, I wouldn't bothered myself and waste my time telling to someone who never wants to listen. If they will soon to get back to me and ask again, maybe I'd change my mind.
Many people just never know about Ponzi and so their limited knowledge about Bitcoin will brought them thinking also about as the same to Ponzi scheme. If they could just spend time learning both of them (Ponzi and Bitcoin), they'll be likely to change their mind.