The simplest way this could work is in retail. He can find a product that he could buy at X and sell it for 2X. If he can sell all the product in the month, he can generate $20K on $10K. Of course, there will be other expenses and taxes.

Whatever business that can double your capital every month will require a ton of hustle or preparation. Other than DeFi, there aren't many such opportunities that don't already have somebody doing it. Any such opportunity is sure to attract competitors, which will drive down the price.