Most of the comments here are to the point. I thoroughly respected the choice of a co-worker to have her 18" Smallmouth mounted. It was her trophy. Meanwhile I have released larger fish without a second thought.
As the owner of a 1970s skin mount (24" Brook Trout), I want to make some comments.
Though the trophy size alone could justify the kill, I am not sure I would do it today. First C&R was not widely practiced back then anyway. Second, replica technology was not a choice at the time. Today I would have had more choices, and the practice of CPR Is more fully understood. Long ago Lee Wulff said,"A good gamefish is too valuable to be caught only once." That tips the scale toward releasing the trophy for the future of fishing and anglers.
Then there are two sides to the biological argument. On one hand, the fish in question may or may not be past its prime. That is hard to determine on the spot. On the other hand the size is a good clue that the fish has those big fish genes needed in the gene pool. Erring on the side of caution, one sould let it go, IMHO.
Finally, when I look at my mount, I realize that most of it is a replica anyway. The taxidermist preserved the head, skin, and fins. The body is a paper machet model over which the skin is fited. The fins are attached and the head is rebuilt including artificial glass eyes. By looking at the wall side of the mount, I can tell that the preservation process had left the hide a dull grey color. So the mount was painted to match the colors in the photo we supplied. IMHO, you get a replica either way.
Get some good photos and length-girth measurements and let her/him go to fight another day.