Is she simply insured as a named driver? Has your son allowed his (I hope fully comp) insurance to lapse? Has he considered who would get an insurance payout if the car were written off whilst in her possession? She doesn’t appear to have an insurable interest in the car and would only be able to take out third party liability insurance.
I hope your son informed his insurance company that the car has been loaned to his ex-wife or it may be uninsured. It’s not simply a matter of her needing to be a named driver, the car is being kept elsewhere.
My son is the registered owner of his car. He can also prove he bought and paid for the car. He let his exwife use the car for 18 months since they separated but he now wants it back. His ex does not work or have custody of the child from the marriage so he has the greater need. He intends to contact her and formally ask for it to be returned. If she does not comply can he declare it off road(sorn)?