OP, Apart from the gaffe pointed out in by DWMB2 in the previous post, you've left full details of the PCN reference number and vehicle registration in the copy of the reminder that you posted. You should fix these mistakes ASAP.
If I've understood correctly the individual who is the hirer was not the driver and also didn't receive a notice to hirer. On that basis, the hirer should write to the parking company along the following lines.
Dear Sirs
PCN [ ] RE Vehicle number [ ]
I write in response to your purported "final reminder."
I am the hirer of the vehicle but I was not the driver at the time of the alleged parking event. Therefore, you have no legal claim against me as driver. As you failed to deliver to me a "notice to hirer" that conforms to the requirements for a notice to hirer in Schedule 4 to the Protection of Freedoms Act 2012, you have no legal claim against me as hirer.
I require you therefore, to cease communicating with me concerning this parking charge and to erase my personal information from your database.
Yours faithfully,