A vehicle's registration certificate only states who the registered keeper of the vehicle is. It doesn't give the name of the owner of that vehicle. For example, most company cars will have the employee's name and address on the registration document, even though the vehicle is actually owned by their employer or a leasing company.
The fact that your partner was the registered keeper of the car does not mean that he was the sole owner of the vehicle. The law might well assume that the car was a jointly owned asset, shared by your partner and his wife. It will be up to the lawyers (and/or courts) to rule on the matter.
Until a full settlement is reached, there's nothing that your partner can do, other than to ensure that his solicitor (and the court, if applicable) are aware that the disputed ownership of the car should be taken into account when a settlement is drawn up.
Chris