The drivers detailed on the Certificate of Motor Insurance as ‘permitted drivers’ are the only people legally allowed to use the motability vehicle.
Anyone else that is not detailed on the insurance policy is not allowed to drive your car regardless of whether they have fully comprehensive insurance on another vehicle or not.
As the owner of the Motability lease, you may be held responsible for any damages or losses which occur if such an event was take place. Furthermore, you may be prevented from applying to Motability in the future.