In December 2014 United Airlines passenger Mark Gongol heard such a call, because the aircraft commander had suffered a heart attack and was out of action. Gongol went forward and explained to the first officer that he had plenty of experience on Air Force jets. He helped her divert for a landing by operating the radios and acting as a backup.
But what if both pilots are out of action, and there's no jet-rated pilot in coach or first class? Here's an interesting Quora answer, explaining how a steely-nerved passenger could land a late-model B737 in an emergency, with guidance from air traffic control:
While researching aerospace articles over the years, working with instructors in professional-quality training simulators, I've sampled a variety of jet-powered aircraft, and it was humbling!
One adventure was trying to land a simulated 737 at then-National Airport in Washington. I finally made it, but only after much assistance from a seasoned instructor. He handled the throttles so I could concentrate on the yoke, flaps, and rudder pedals, but it was still quite difficult; a critical skill turned out to be using the trim switches on the yoke. A later challenge was lining up a B-2 bomber with the refueling boom behind a KC-10 air tanker. (That simulator facility at Whiteman AFB had the strictest security precautions of any military installation I've visited, BTW.)
My takeaway: there's no substitute for small, well-timed inputs. In the 737, the aircraft and its engines responded slowly to control changes, so it was easy to fall behind ... and fall to the ground.