James W - those are called "channel lock" pliers, commonly with blue handles. A good choice if you can find one big enough to fit...never seen one big enough to fit my own unions, but would work great if he can get an appropriate size set.
Careful though, as they can easily tear through PVC if they don't have good strong points of contact. Won't cause functional damage in most cases, but can leave clear evidence of your fight.
Edit - I have destroyed a unions with channel locks before on a friend's pool...where they did fit...sheared right through the PVC...so be careful if channel locks are your weapon of choice.