If your credit card has a low maximum (say $2k), and you don't pay interest on the balance you pay off every month, your downside risk is having to pay the card interest, and your upside is passive currency exchange income.

Your acquaintence is going to be very wealthy. Not from this trade, but from the instinct to see the leverage opportunity and take that risk. Sure, bitcoin could go to a small fraction of what it's trading at, and he is out $2k minus whatever he netted on the leveraged trading. The cost to account for it properly outweighs any profits at that level, but it's a scrappy trade by someone who saw the opportunity.

As someone who still mainly makes money with labour, I was impressed anyway.