I would simply don't care at all, if they think they can get rich through gambling, then they're stupid.
As long as they sign using my referral and I have warn them, I don't care about other thing. Problem is inevitable, but you can control how you react.
I think it would be nice to start caring about others instead of being self centered, warning them is not enough since you're the one who introduced them into it at the first place, atleast you should ensure that they're doing the right thing with the update you've shared to them, i don't buy the idea of calling them foolish cause different people got different challenge and you never can tell the challenges they had been going through before you introduced them to it.
Left for me I'll feel the guilt if i introduce a close friend into gambling, maybe for referral bonus or so and they end up being addicted to it, moreover the last thing I'll do is advise someone to gamble using Bitcoin, they should do that with other Cryptocurrencies but definitely not Bitcoin.