All good suggestions. But if your friend tries to lock his wrists and can't get 'em firm enough (it DOES happen and it's nothing to be ashamed of as long as you're ready to fix it) there are lots of ways to make wrists, forearms and grip stronger.
First of all, those grippers someone mentioned are probably good enough for your friend if he can't lock up enough to shoot a G17 well. Once you get a decent level of strength built up,
http://www.ironmind.com sells REAL grippers
-- the trainer models start out at 90 lbs. There are only 4 models. If you can close #3 with one hand you get to be a "Captain of Crush" of whom there are only about 10-15. If you can close #4 hurry and call Ironmind because Joe Kinney is the only man known to be able to do it so far!
Newspapers are another good way, crazy as it sounds. If you spend five minutes on each hand each day doing nothing but laying a sheet of newsprint flat and crumpling it up with one hand, you'll end up with a pretty good grip.
You can also make a good "roller" for cheap. Tie a rope or twine to the center of a short piece of pipe, then tie a weight of some kind to the other end. Then you just slowly wind the weight up and down by turning the pipe using both hands. The higher you can stand to make the weight hang farther, the better.
Farmer walks will also help. If your friend lifts weights it'll be easy. Just pick up a pair of heavy dumbbells, get your grip the way you want it, and start walking. When you absolutely can't hold 'em up anymore you're done. You only need to do this a couple of times a week.
If you want to know more, check out
http://www.ironmind.com and
http://www.cyberpump.com . Each has good info on grip strength.