I tried calling a few car rental companies to ask about the financial responsibility of an
additional driver, but all were reluctant to give me any information without me having a specific contract number for reference. There is not much information on the different websites about specific responsibilities of an additional driver.
Bascially, when someone rents a car, they assume full finacial responsibility for the vehicle, regardless of who is actually driving the car.
I dug up an old contract that I had from a Budget Rental that my husband took a few years ago, and found this clause in the contract.
(in this contract, you, refers to the renter)
Paragraph 11 Section 1
Loss Damage waiver, Damage and Loss of Property
11.1 Subject to this clause 11, You are liable:
A. for the loss of, and all damage to, the Vehicle; and
B. for all damage to the property of any person:
i. which is caused or contributed to by You or an Authorized Driver; or
ii. which arises from the use of the Vehicle by You or an Authorized Driver. Remember that references to the "Vehicle" include all of its parts, components, accessories and contents (see the definition of "Vehicle" in clause 2).
I would get together with the renter and go over the rental contract and the additional driver form that you signed and carefully look for information about the renters responsibility, especially with regards to damage and additional drivers, and if there is any information listed specifically about the financial responsibility of an additional driver.
The renter may also want to contact the rental car company and request specific information from them about the financial responsibility of an additional driver.
Basically, if the renter signed something that stated that they would take full financial responsibility for the vehcile, your insurance company is under no obligation to cover it.
I'm sure this isn't the answer you were hoping for, but I hope it helps, regardless.
Pages from this site, relating to this topic:
Car Rental Additional Drivers Overview
Company Specific Additional Driver Policy
Car Rental Agreements