I agree that once he notifies them of the problem, his obligation has been met. He can't fix the problem himself and stopping the usage isn't an option. Now, the park may come back with an amount owed that he may be responsible for that they will have to justify the amount.
To me, it is much like a situation I had one time where I had a store clerk argue with me about the bill that I gave her to pay for my purchase. She gave me change as if I gave her a $20, but I actually gave her a $10. I know it was a $10 because it was the only cash I had on me at the time. When she handed me the change, I tried to hand her the $10 bill back and told her that I had given her a $10. She got very defensive and told me in no uncertain terms that I had given her a $20. At that point, I felt I had met my obligation and left with a clear conscience. I'm sure she thought about our conversation at the end of the night when her register was short.