if they already do that i think we wouldn't judge him, but if they haven't do it i think you can tell him about the risk, about the car that you sold will gone if the bitcoin price is drop or bitcoin is not popular anymore in the future
Even I don't think that Bitcoin will lose its popularity soon. There are still lots of novices who buy their first crypto as they believe in it. And for now there's no reasons to forget about it. I'm sure the price will not reach 500 000 but it will rise for sure.