For a completely untrained person it would be hard and would take hours of practice, so a lot would depend how much fuel the plane had.
In theory it is possible, as long as you have a long runway, because you can land it on a really shallow glide slope, which takes a lot of potential problems out the equation. As long as the guy has it lined up right, configured correctly and is going the right speed, the plane will more or less land itself. It's just a question of how many times he has to go around before he lines it up right.
Don't forget the Barefoot Bandit who stole a bunch of planes and "landed" them multiple times successfully using only the information in the operator's manual. Not pretty landings, but, hey, he landed and walked away each time.
------------------- ** FOLLOW UP **
I asked a senior pilot/instructor/examiner at American about this. He said the following:
(1) Stewards are not trained to take over in the event of such an emergency
(2) On many flights there are pilots flying in a jump seat or making a transit and these could take over if they are present.
(3) In the unlikely event that the crew was incapacitated and there was no other regular pilot on board, the flight attendants would ask among the passengers if anyone has a pilot's license. In this situation the disadvantages of "inducing panic" would be irrelevant compared to the value of having an experienced pilot flying the plane.
(4) If an untrained person had to fly the plane, it could probably be done successfully in most cases using the automatic features of modern commercial carriers.