I have been in the property and auto insurance industry since 1980. Without seeing the actual contract, I am not in a position to provide advice on exclusions or limitations relating to drivers or anything else. It is certainly possible that only specific drivers are covered so be very cautious.

Make inquiries directly to the insurance broker involved or, if necessary, the insuring company. Make sure there is full disclosure of the circumstances without misrepresenations. Having documentation is highly recommended. An email would do just fine.