When he asked for syn gut what was in the racket? If it was not a syn gut you carried you should have tried to put in something similar to what he had unless he didn't like the strings. Sounds to me you didn't do anything to see what the customer really wanted unless you are leaving something out. To me you're both to blame and should both share the cost. I would offer 1/2 price on the service and full price for the strings.
Now when he asked for Xcel that's a different story? Did he mentioned a specific tension? Did you talk to him about what he likes? You don't want him back in 2-3 weeks unsatisfied again do you?
Everyone has the right to be stupid... it's just that some people abuse the privilege