You can just tell them the risk involved in engaging with those Ponzi scheme companies.  It is up to them to listen to your advice which I hope they do.  If they do not then at least you tried your best to warn them.

I also come in contact with people who invest in Ponzi scheme.  Since they had already invested, I just told them to wait for their account to mature and withdraw their fund ASAP since I told them that the system is doomed to collapse at one point in time.  They don't listen to to me but instead from $1000 investment they add another $40k, yes they get greedy when they receive their first payment.  After a year, I asked if they were able to get their money and profit, and they told me they were into it and filing a case to get their funds.  I did not comment anymore to avoid adding salt to the injury.