Expensive lesson. Next time you or your friends park on private land, check the terms and conditions of parking there.
However, if you don't want your friend to pay anything to UKPC, then you will have to follow the advice you receive here. No appeal is going to be successful.
This will be won, eventually, after they issue a county court claim and it is defended using the defence we provide here. You are looking at a process that is going to take anything from 8-12+ months to complete.
Is your friend willing to fight this (lesson learnt) and not have to pay UKPC? If not, I will nt bother wasting my time.