UKPC and Debt Recovery Plus Ltd

My elderly Dad received a Parking Charge for £90 from a BPA member Private Parking company for allegedly not displaying a valid pay and display ticket in a Retail Shopping Park. Since the letter was sent over a month from the alleged offence he would not have kept the ticket so cannot prove one or the other. He disputed this as he believes he had got a ticket to cover the 2 hours free parking and the photo in the letter is of the rear of the car, however the company say that it was issued correctly but reduced the penalty charge to £50 if he paid within 35 days. Still annoyed about this I looked at various pieces of advice on the web which seem to suggest just ignore them and they will eventually go away. He has now received a letter from a debt recovery service asking for £150. I would appreciate advice please on whether to continue to ignore letters or is there action he should be taking?