Well, if he hasn't actually flown in 20 years, he cannot exercise any privileges of his pilot's license. It hasn't expired, per se, but then it's functionally useless.
It's impossible to LEGALLY "update" without flying. Every two years he must fly with an instructor who will "review" his competency. There's no minimum amount of flight time prescribed; it could technically be once around the pattern...a very wide pattern. But an instructor must sign an endorsement that he has been given a satisfactory FLIGHT review.
He may keep it "updated" without flying by having a less-that-scrupulous, good-buddy instructor just pencil-whip the endorsement without demanding the flight. But this is not legal.