Well there's nothing to tell you it's a car park, so you couldn't possibly know that you're in an off-street car park subject to an off-street parking places order.
I'd suggest your friend waits for the notice to owner, the notice to owner will demand the full amount but as long as it is challenged within 14 days the discount is almost inevitably reoffered, so don't worry too much about the discount.
In the meantime I've already requested the order form the council. Please can you confirm your friend is the registered keeper and he has the V5C, and also that the address on the V5C is up to date?