Learn French: Est-ce que tu vois les clients avec qui il s'entend ? - Do you see the clients he gets along with?

In English, we often leave the preposition at the end of the sentence: "the clients he gets along with." French never does this! The preposition must go right before the relative pronoun. Since we are talking about people (les clients), we use the preposition avec followed by qui (meaning "with whom").

Wait, doesn't entendre mean "to hear"? Why does this translate as "gets along"?

You are right that the base verb entendre means "to hear." But when it is used reflexively as s'entendre, the meaning changes entirely to "to get along" or "to agree." So il s'entend means "he gets along."

Could I say avec lesquels instead of avec qui for "the clients"?

Yes, you could! les clients avec lesquels il s'entend is grammatically correct. However, when the relative pronoun refers to people, using a simple preposition + qui is much more common and sounds more natural in everyday spoken French.

Why isn't qui il shortened to qu'il?

The relative pronoun qui never drops its i, even when the following word starts with a vowel or a silent h. It is only the pronoun que that shortens to qu' before a vowel. Therefore, it must stay qui il.