With a bit of luck, your sister hasn’t appealed to Horizon yet. If she’d sent what she’d sent to YPS, she’d have blown the easiest way to get this cancelled.
Tell your sister to never, ever, ever, ever, ever, identify the driver unless advised to do so. By blabbing the identity of the driver, inadvertently or otherwise, is to throw away the most likely best reason to get a PCNcp cancelled.
No one is suggesting lying. It is a simple fact that the keeper of the vehicle is the person who has received the Notice to Keeper(NtK). The driver is unknown to the operator. Only the driver is liable, unless the NtK is fully compliant with all the requirements of PoFA. The Horizon NtK does not invoke PoFA and so the keeper cannot be liable unless they were also the driver and blab that bit of info.
There is no legal obligation for the keeper to identify the driver to an unregulated private parking company and the keeper should decline to do so. Tell your sister to appeal with the following, verbatim:
I am the keeper of the vehicle and I dispute your 'parking charge'. I deny any liability or contractual agreement and I will be making a complaint about your predatory conduct to your client landowner.
As your Notice to Keeper (NtK) does not fully comply with ALL the requirements of PoFA 2012, you are unable to hold the keeper of the vehicle liable for the charge. Partial or even substantial compliance is not sufficient. There will be no admission as to who was driving and no inference or assumptions can be drawn. Horizon has relied on contract law allegations of breach against the driver only.
The registered keeper cannot be presumed or inferred to have been the driver, nor pursued under some twisted interpretation of the law of agency. Your NtK can only hold the driver liable. Horizon have no hope at POPLA, so you are urged to save us both a complete waste of time and cancel the PCN.
