If you were in my shoe, what would you've told a person that says bitcoin is a ponzi scheme created to scam people?
None. I will not do anything and I will not say anything to him.
You said in your post that he already invested into crypto with the help of other people. Probably that the person you are referring with is just trolling and possibly doing what the rich man and institutions are doing (spreading FUD's in order for the price to go down then they will enter at a lower price).
Anybody can say anything to Bitcoin and TBH we are not obliged to invest into it. They can spread FUD's into it but I would say good luck to them especially this time that many already know and investing into it already.