I agree with Greg. We lefties tend to be more ambidextrous naturally. We are also more practiced at learning right-handed actions than the rest of the world is at learning left-handed moves. The second is a matter of survival in a right handed world.
I doubt your server will have much trouble learning to put a plate down with his right hand. It is, after all, a pretty easy move.
Seraphim -- the off-hand cut is made with the eyes and the fork. Set the fork parallel to the cut, look where you want the knife to go, and with very little practice it will start going there. As long as you try and steer the blade with your hand, wrist and elbow you won't be successful. You're distracted by the unnatural feel of the knife in the hand -- don't be, your hand will take care of itself. Just watch the cut.