I had to check the internet to see the car that cost $28milloin and I discovered that the Rolls-Royce Boat Tail cost the amount. I don't know what your friend does for a living, maybe he is a professional footballer or a celebrity? I have read so much about some rich scammers or drug lords who could afford such luxury cars. I will assume that your twenty years old friend didn't work hard for this money that's why he could splash such an amount on a car.
Investing in Bitcoin is not by force but by choice. Your friend has the freedom to buy whatever he wants because the money belongs to him and not you. You can only advise him but you cannot force him to buy or Invest in what he wants. Friendship is not also by force but by choice. You are an adult and should choose your friends. If you think that because of his extravagance, he shouldn't be your friend, it's your decision.