The signs say clearly 'Customer Only Car Park' and elaborates with 'You must remain on site at all times your vehicle remains in this car park'.
So perhaps having receipts for the visit(s) isn't so ridiculous.
What has that got to do with the price of eggs in China? The PCN is nothing to do with whether the driver was a customer or not.
The term "Customer Parking Only" is ambiguous, and its interpretation will vary depending on the context and the intentions of Haruphey Shopping Centre or Ocean parking. Simply visiting the shopping centre, whether for window shopping, meeting someone, or buying coffee, should qualify someone as a "customer" because they are utilising the facilities provided by the shopping centre. Even if no purchase is made, the visitor is still engaging with the premises, which could reasonably be argued to meet the "customer" requirement.
However, if some operators or shopping centres want to adopt a stricter interpretation, expecting visitors to be actual paying customers of one of the outlets, this should be enforced through terms on parking signage or receipts required to validate free or permitted parking.
If no specific definitions are provided by the signage (such as requiring proof of purchase or validation), a broad interpretation like window shopping or meeting someone for coffee would suffice. As the parking is enforced/managed by Ocean Parking, they need to be explicit if they intend to limit "customer" to paying customers only. Without that clarity, you could argue that any visitor is a customer under common-sense use of the term.
It would get even more ridiculous if, let's say, the driver remained in the vehicle but a passenger sent into the shopping centre. Would the driver have breached the terms of the contract?
Any appeal against a PCN on these grounds, it would be useful to emphasise that the signage was unclear and that the driver was indeed utilising the shopping centre's services, even if it was not through a direct purchase.