If he's finding them on the deer, the deer must be dying, which means he's doing something right.
In my experience, most slugs are recovered inside the deer, still intact, but very well mushroomed out.
I have a baggie full of recovered slugs at home and will try to remember to get some pictures.
However, these are almost always recovered on the offside of the deer, just under the hide. So it is complete penetration, just not exiting the skin on the off side. I've recovered slugs on deer from 10 yards out to probably about 60 yards. None of my slug shots have been far. And most are very close, due to the thick stuff I primarily hunt.
-Jake