You say the driver was not the registered keeper. Whilst the driver is liable, there is an attempt to use PoFA to hold the keeper liable. Technically, they have not fully complied with the strict requirements of PoFA in that they have failed to invite the keeper to pay the charge as required by PoFA 9(2)(e)(i).
Not that ECP will acknowledge that. The keeper will more than likely have to lead a POPLA assessor by the nose to that fact, together with all the other POPLA arguments, poor signage in particular. Many ECP POPLA appeals have been successful on the signage argument. Their signs are difficult to read, probably why the driver never noticed them.
Has the keeper tried Plan A yet? Have they tried to contact the garage/landowner to ask them to get this cancelled?
Any initial appeal should be by the keeper, only as the keeper. There should be no divulging the identity of the driver. ECP tend to follow the well trodden path of farming these out to DCBL to try and scare the keeper into paying and then using DCB Legal to issue a county court claim which, when robustly defended using a template defence, is eventually discontinued. However, that is far down the line.