Before deciding on the action, we need to establish that the comment was really "inappropriate " we need to dive deep into the context of conversation if we really want to find out what was wrong with the employee.
People sometimes ask questions their respondent have nothing to say about and they feel they must keep the face by mocking the person asking the question. This can actually be rather annoying. So the lesson the employee must learn is to "abstain from demonstrating that he is smarter than customer".
For example here on SE people who answer question sometimes get a reaction from the person who asked the question: "Are you saying that you're smarter than me, that you know better or maybe you imply I'm more stupid than you?" This is a very common theme, constantly repeating itself and since it is distressing, you might consider teaching him to never speak in public, unless he made sure
- The client really is interested in receiving an answer
- The answer specifically deals with the issue of "who us smarter" and indicates that it's not the employee
You might want to specifically train him answering to stupid qurstions/remarks in respectful manner by asking stupid questions and making sure his reaction is appropriate.
I think this concept is called "intellectual humility", it may be not popular these days when everyone actually must outsmart others, but since his role is humble and lowly, he must never get out of it.
Since most people (not sure but I estimate about 80%) are brought up and made believe it is important to never appear smart/knowledgeable, so they despise this characteristic in others.
This is especially relevant if the employee happens to be an immigrant and in this case he must be especially careful to make sure that he doesn't say anything smart.
This may be controversial and speculation, but WHO we are to trust any sources other than some popular news outlets?