Can a passenger land an airliner with help from air traffic control and instructors? This is a very unlikely scenario and has never occurred in the history of aviation. We can only speculate as to how such an incident would play out. There have been occurrences where passengers have been talked down. Both involved pilots losing consciousness in small aircraft (four-seat general aviation types) that resulted in a successful talk-down or, in one case, a successful crash landing that resulted in the survival of the passenger (the pilot did not survive the medical event that lead to his unconsciousness).
First, let’s assume, in a post-9/11 world, that one could even access the flight deck. If that were not a problem, there may be a more qualified flight crew member aboard (flight attendant with a license or a dead-heading captain or first officer) or a former airline pilot aboard that could assume command. That would tip the scales in favor of success. If one only has general aviation experience in light aircraft, or none at all, the probability of success decreases. However, having someone in control with at least basic knowledge or one that can be instructed, is far better than no one at all. A scenario such as this would also require a lot of assistance from the ground as well. Communications may be an issue too as one would need to be able to work the radio or at least figure out how to use it in a timely fashion.
It remains to be seen if something like this will ever occur (hopefully not). If it does, it would likely take a lot of time, communications, and practice before a landing was actually attempted with an unskilled passenger at the helm. I also hope that if something like this happens, the outcome is successful.