We need to see your partners letter. Dates and specific circumstances are important. In the letter linked to there appears to be no mention of POFA so as long as the drivers identity is not revealed they can't hold anyone liable for the charges, certainly not the keeper. Thats a stronger defence than quibbling over the term motorist, although it does look like the wording on the letter was written by an amateur, not sure how you can issue a charge to a vehicle.
