Ignore your LHBS guy. My guess is he assumed you would put the hops in the keg loosely, rather than contained. If you're going to keg the beer, you want as much fresh aroma as you can. Use a nylon hop bag, and loosely tie it to the dip tube of your keg - this will prevent it from clogging your pickup, especially if you are using pellet hops. I usually dry hop with leaf hops in the keg, but both will work if you use a hop bag.
Keep me posted!