His estate pays them back when he dies. Inherited shares have their cost basis reset to the current value, so if his heirs sell them immediately there's no capital gains to be taxed.

The loan will be cheap but not free. The problem with shares is that their price can go down to zero in case of company bankruptcy. So there will be premium for that risk.

Not really. His loan will be way way way way over-collateralized, and like I said, he can even hedge it perfectly. His cost will likely be about the RFR.