As a co-owner of the vehicle, he can be held accountable for whoever drives that vehicle. That being said, he needs to conatct his insurance company of this recent development. If the vehicle stays at her residence, he may have to remove her and the vehicle from his policy. Many insurance companies will not insure vehicles from more than one household. (Unless, it is a student risk attending college in the same state from which the policy originates.) She will probably have to purchase her own policy for this vehicle. Since he is a co-owner, it would be in his best financial interests to sell the vehicle to her outright. Or, make sure she has very good insurance coverage, follows the conditions of her new insurance company, and be scrutinizing of anyone driving that vehicle (assuming it is fine with her insurance company).