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
Car is already sold and it is his decision. Tell him that diversifying investment can help to lessen the risk in bitcoin. If you are afraid bitcoin lose popularity then find another where you can believe on that can give profit in the future. However, I do not think bitcoin to lose popularity because it is the foundation of all other digital currencies.
Totally agree, I don't think that now we have alter coins that may be more reliable than Bitcoin is. Probably that's why he chosen btc and not something else.