What's the issue? The mortgage is going for a rental property. Income will be provided by the rent (which should be at least 1.25 times the mortgage payments to cover void periods). If the mortgage is unsustainable, the rental property can be sold in order to redeem the mortgage. It doesn't affect his own property.

The issue would be if there was a property crash, and the rental property was left with negative equity. However, that is an issue with the buy to let market as a whole. Assuming property continues with its mad growth, it is more likely that heirs are left with an asset than a debt.