Having quickly read the thread it looks like they didn't demand the payment, they merely informed your wife of their policy and she paid up. If, when told of this charge, she could have told them that she was not happy and have left without payment for either the missed or the rebooked appointment?
In this case, the position of making a claim would have been left with the shop, and in the absence of any contract, would have not been particularly easy or financially rewarding enough to pursue.