Does she have any current points?
It is bad news I am afraid which will not be the answer you want and certainly not the one I will want to give. When you had the initial notice (it is called a Section 172) it would have stated on it that it was to be completed by the owner of the vehicle and should not be handed to the driver. The owner has an obligation to complete and return the document. As you know you did this but they wrote back and said that it should be completed by your wife. This was not done and as such the law was not complied with. The law states that:
(2)Where the driver of a vehicle is alleged to be guilty of an offence to which this section applies—
(a)the person keeping the vehicle shall give such information as to the identity of the driver as he may be required to give by or on behalf of a chief officer of police, and
(b)any other person shall if required as stated above give any information which it is in his power to give and may lead to identification of the driver.
As she did not complete it and just lost it, that is not a defence. Therefore she will have to plead guilty because the form completed by her was not returned as required by law. The bad news is that this will be a fine and 6 points which is mandatory. Sadly there is no way of getting out of this as you accept receiving the form and even having a reminder. The law is very clear, the owner has to send back the completed form nominating the driver.
If this fully resolves your question then please take a moment before you leave and rate my service, it only takes a second and is an important part of the process. Otherwise if you need more information or help then please click reply. Alex