It’s not his or his wife’s mistake but the parking companies. A ticket was issued, their money taken, that the ticket was not fit for purpose - how is that their fault? Don’t forget that the parking company also has their money. The sums are irrelevant, the principle isn’t.
I know someone else who got parking ticket in same place Mrs MB did for same reason, used my defence as above and also won as the parking firm didn’t attend court. They also went for their costs. I think it was £300 odd quid I got out of them for about 20 hours of my time to prepare my case - I didn’t even have to attend court as the firm never paid their fee so the case was thrown out - apparently it is/was a trick they use… if you don’t defend they pay the fee and win by default if you do defend they don’t pay the fee and the case is thrown out. I just filled in a court costs form and that was a formality to be awarded costs. I also got plenty of threat-o-grams before they went for court.