My understanding is he would still have to pay back the loan eventually. But the 2 bitcoin show the lender he is responsible with his money and can/will pay back the loan. If he doesn't they get to keep it.
With a deflationary currency he might not even have to pay interest. 🤔 Everyone could still win, the lender gets back their initial investment, now worth more purchasing power. The borrower gets to make the investment on their own time table.