This is the law as far as I can gather:
When you park in a private car park, you enter a contract with the owner (I will pay X for parking for X hours). So by not buying a ticket your ds has broken the contract he entered. He is therefore liable for the cost of parking overnight - whatever that is.
The police will not pursue him to make him pay, or take him to court.
The private company could take him to small claims court. If they do this for the cost of the parking they had, plus their (reasonable) costs in taking him to small claims, they are likely to win.
They are not allowed to slap a huge "fine" for non-payment on.
He doesn't have to pay anything until they take him to court.
If they do take him to court, he risks getting admin costs and court costs added on to the original cost of parking.
Personally, in his position, I'd pay the money owed (cost of parking) now, claim a mistake (I forgot to get a ticket) and then let them take me to court if they wanted to.