Listen carefully to understand their point of view and acknowledge you heard and understand their concerns. People want to be heard and understood and it goes a long ways to calm the person down. From there you will want to find solutions that are agreeable to the store's interests as well as the customer's interests. When in doubt error in favor of the customer, but never make a fellow co-worker look bad. Keep your tone calm and respectful at all times. Never take it personal and never get defensive, nor snarky. You can always vent later, in the break room, but never on the sales floor. Extreme customer situations can include security, but this is not typical and should be the last resort.