The driver entered private land that was signposted as being so and that there are terms and conditions if they want to park there. If the driver chose not to read the signs, that is not an excuse. They are there to be read.
The signs form a contract with the driver. One of the terms are that the car park is ONLY for the use of McDonalds customers. If the driver was not a customer, they breached that term and therefore contractually agreed to pay the £100 charge.
What is there to dispute? I don't normally stick up for these leech companies, but you have clearly admitted that the driver breached the terms and conditions, so why do you think they should not have to pay the consequences?
As for the legal points, what you have shown us is a Parking Charge Notice (PCN) issued as a postal Notice to Keeper (NtK). It appears to be addressed to a company, not an individual. Is the vehicle leased or hired from a lease/hire firm or is it addressed to a company that is the registered keeper?
What evidence is there that the driver left the premises. The only evidence on that NtK is a photo of a vehicle parked. I see no evidence of anyone "leaving the premises". Whilst the signs may state that the car park is only of the use of customers, the only person with any liability is the driver. What any other passengers do is of no relevance.
How are the boundaries of the "premises" defined? How would a driver know that they are leaving the boundary of the premises?
These "leaving premises" PCNs are almost impossible for them to prove and if it ever went as far as a court hearing, they would stand almost zero chance of proving their case. That does not mean that they will not try and take it all the way in the hope that you are low-hanging fruit on the gullible tree and can be intimidated into paying up out of ignorance and fear.
There are other similar cases for this location already on the forum and you should search for them and follow the advice given in those.
Here are just a few:
https://www.ftla.uk/private-parking-tickets/genuine-mcdonalds-customer-fined-for-leaving-the-site-seeking-advicehttps://www.ftla.uk/private-parking-tickets/pcn-met-parking-occupants-left-premises-mcdonald-shttps://www.ftla.uk/private-parking-tickets/met-parking-services-occupants-left-premises-mc-donalds-leytonstonehttps://www.ftla.uk/private-parking-tickets/met-parking-charge-at-mcdonalds