As usual, I'm in complete agreement with Art.
I can remember times in the past where I had hunted hard and not seen a deer for some time, only to get really flustered about finally getting to see my quarry! "A deer! A real, live, honest-to-Gawd deer! Holy smoke! Now I better shoot quick!" Heh.
The best answer is Art's desensitization concept. Get the man used to the sight of the critters. Best is to get out among 'em when not actually actively hunting, ahead of time. But life is imperfect, and some folk can't get out to the hunting fields more often than the week or so that they take off in the fall. So what to do?
Well, nygunguy has an excellent tip with the deer targets idea. This works for two reasons: it trains the hunter to practice aiming for the kill zone and not just "shoot it in the deer," and it puts a picture of a deer in front of the hunter, so that he can get used to it. Heck, I've even been known to cut out cardboard cutouts of a deer, so that I've got a free-standing target that gives sort of a 3D effect. There's a tremendous difference between shooting at an inanimate high-definition target on paper and a low-contrast, mostly-camouflaged deer, even at close range.
Finally, there are two other things that the hunter can practice at the range: field positions (NOT from the bench) and shooting under pressure. If it's at all feasible, have him, when he checks his targets, run at a dead sprint to the target stands, mark the shots with tape, and sprint back to the firing line and quick fire a 3-shot group from field position while huffing and puffing. Even better if you run along behind him, yelling at him to hurry up. This will humble a LOT of us, you can be sure. Get him to practice slowing his breath and calm down enough to make a decent hit. If he finds that it takes him 20 seconds to calm down enough to make a 6" group from 50 yards from a sitting position on a deer target, then he's learned something pretty valuable. If he learns that he simply can't hold 3 shots on a 10" circle on a deer target at 100 yards after this excercise even after a minute of settling down, then he's learned that he'll need to pass up that shot and move up.
Personal story:
My first long shot that I ever made on a deer (300+ yards) was made on the last day of a deer season in which I had hunted 9 separate days without seeing a thing. I had literally not laid eyes on a deer the entire season, except at night while driving back. Lots of sign, but no deer. When I finally found a group of them in the large field that I went to early that morning, I got entirely too excited-- I was young, and also felt like I had a lot invested in this one last chance. I took a sighting on the big buck out there, and watched my crosshairs bounce feverishly to the lub-dub of my heart as I struggled to hold them on him at almost a quarter mile away. I stalked out in the field in a belly crawl and closed about 100 yards before I realized that the buck was going to leave the open field for the cover of brush soon. I took a careful rest across my backpack, knocked off the safety... and almost blew it entirely. If I had shot then, I would have at best missed that buck, and at worst wounded it. I stopped, took a couple of deep breaths, let 'em out, and relaxed. Far better to let the buck go than to blow the shot. I tried to sight again a few seconds later, and found that the cross-hairs really had settled down a lot. As the deer started to leave the field, I made the shot, and though it ran, I knew --knew-- that the shot was good. [And so it was.] Over the years, I've occasionally had to pass up on a shot when I knew I wasn't settled enough for it. [shrug] Beats hell out of wounding it.