This situation may be too hypothetical to give a direct response.

If the driver of a car at fault (that is not on the policy or not given permission) has an accident, the insurance company of that car would need to determine if coverage would be afforded.

If it is not afforded then the claimant could claim it under their own policy. This would typically be claimed as an uninsured driver and the insurance company may require a deductible to be paid (if applicable coverage is used).