By letting them know that I understand them and willing to make it right with them

Letting the customer know that we are sorry and we will fix the issues.

Keep them calm keep yourself calm that’s all the problem don’t escalate and reassure them that the issue will be fixed sorry for the inconvenience just count the food and maybe a gift card to ensure they will come back and visit us again

I would stay calm, explain my side of the situation and try to calm then down. If this didn't help, I would notify management.