It doesn't matter whether the customer is right or wrong. If I saw a customer getting treated like that, I'd leave in a heart beat. The manager needs to understand that he was not only speaking to one customer but was speaking to all his customers. If the manager continues to behave this way (like a child instead of a pro) he will not stay in business. The manager has shot himself in the foot because not only did he loose this person's money, he probably has lost other customers by his behaviour. This is not a recipe for success as a business.