There are a couple to times when it generally doesn't matter. For Hertz anyway, any rental for business means you can let a coworker drive the car and they don't have to be listed. Your spouse or domestic partner does not need to be listed when you rent the car either, at least when you're Gold.

Other scenarios require the person to be on the contract. In some cases that also costs extra. So that is the other part of the equation if the unauthorized driver is in an accident.

This is a problem from an insurance standpoint whether you buy the Hertz insurance or not. In your specific scenario, it's quite possible your dad (or others in your family) would not have any coverage on the car should he damage it. He is not the renter, not on the contract. Who would he go back to if damaged? Whose insurance? Nobody. Not his contract, so not his credit card. Not his contract and not his car, so why would his car insurance pay? You weren't driving, so not your credit card. You weren't driving and it's not your car, so why would your insurance pay?

Get the picture?

Useful things in this instance are a) add drivers to the rental the official way if necessary and b) possibly, something like Amex Premium rental car coverage, which covers damage to the rental car, as the primary coverage (so , even for additional drivers. It only costs $20-25 per rental unlike the pricey Hertz LDW. Because even if an authorized driver, you might still have an insurance issue.

It's something worth asking an insurance agent about at the very least, how the coverage would work in these situations, if you don't want to simply mitigate the liability.