I would listen to the customer and ask him/her what upset you the most about your experience? or what is upsetting you right now? Listening, I would then repeat their complaint to them, letting them know I understand.
I would tell them I will do my best to assist you. If there is something against company rules then I would let them know that and encourage them to file a formal complaint or talk to the Supervisor.
If allowed to comp a meal or beverage I would do so.