I guess one approach would be to have a few “seeds” of knowledge to sprinkle into conversations.

So in this case, when he suggested to use a CC for fun you could say something along the lines of “well, if I can’t pay cash for it now I fail to see where expecting to have cash to pay for it later is a better choice”. And then drop it.
Just dropping food for thought would be my approach. If it piques their interest enough to have a real conversation about it, then so be it. We have a friend whose financial choices have just driven us nuts over the years. From car purchases, living large and spending frivolously all while complaining about her salary, etc, it has always driven us insane. I suspect she knows our thoughts, but we keep them to ourselves. I do however make our personal situation quite clear – when we bought our last car, for example, I made it clear it was paid for in cash.
Anyway, good luck with this. Painful to watch others make unwise choices…