now I have to send him some money to buy some food.
You don't have to do anything. You're doing it because you want to.
I have friends who are shopaholics and spend too much. Sometimes they get a bill and want me to pay for it and I don't do it. I tell them to sell that new dress or shoes they bought and pay the bill. It's not like they need 20 dresses right now, but they have to pay the bill now.
Looks like you need to teach your friend about priorities before you teach him about saving money.
Friends are said to be shoulders to lend on when one is in a tough period. But I think its case to case basis too like what happened to your friend. He knows already the risks of investing all in with bitcoin but still he did not listen to you so maybe whatever he is facing right now should serve a lesson to learn. So my advice is never lend him money, let him find the solution to his own problem. But you can give him foods if you want so he can feed himself despite of the consequences he is into right now.