What would you do if you overheard a store employee making fun or talking bad of one of their customers behind his/her back?

I was at Home Depot the other day, and while waiting in line, I overheard one of the employees mocking and making a derogatory comment about a customer that had just purchased something before me. He mumbled to his other young co-worker, “That guy was so gay. No wonder he bought a flowerpot. Hahaha.” Fortunately, the customer had already left, so he didn’t hear what the employee said about him. Both employees stopped smirking when they saw my astonished look. I gave the young guy a disapproving look and shook my head, and he just sauntered off. What would you have done, if anything? Quite frankly, I felt I should have said something, but the incident caught me off guard. I know the talk was between the two employees, and the young guy made the comment quietly (thinking no one else would hear it), but it was just audible enough for me to hear it. To cover his butt, he could have said to me “I didn’t say anything like that,” or “Sorry, I was talking to my buddy, not you.”

How about if you were the customer they were making fun of, and you overheard them whispering something bad about you?