“A great many people came that day to express their solicitude, among them was the wise man.”
The original construction above is actually a run-on or fused sentence—a comma splice, in fact—because a comma is inadequate for punctuating the two clauses. You are therefore correct in spinning off the clause “among them was the wise man” as another sentence; it makes the statement much clearer. But this construction would be advisable only if the statement is a stand-alone sentence—meaning that it’s not followed by another sentence as part of an exposition or narrative. In such situations, spinning off a modifying clause as another sentence could disrupt the flow of the exposition or narrative.
A simpler fix is to replace the comma with a semicolon: “A great many people came that day to express their solicitude; among them was the wise man.” But there’s a much better construction and stylistic alternative than this: converting the clause “among them was the wise man” into an absolute phrase modifying the noun phrase “a great many people,” as follows: “"A great many people, among them the wise man, came that day to express their solicitude.” This way, no problems of continuity arise because every idea in the original statement is contained in the same sentence. Another sentence can then be added to the statement without any disruption in the flow of the exposition or narrative.
