It's interesting that my comment gets downvoted for stating a simple fact about training and knowledge in air traffic control (ATC).
In 2008, there was even a fatal accident in Zürich, Switzerland where the inadequate training of air traffic controllers with regard to piloting an aircraft contributed to the accident (although it was not the cause of the accident):
'The following factors created conditions for the accident or favoured its occurrence:
[…]
* An inadequate understanding within air traffic control of the significance of faults and emergencies on single-engine light aircraft flown by a single pilot in challenging weather conditions.'
Swiss air traffic controllers do 25 hours of flight training in small aircraft, so they have a basic idea of piloting an aircraft. In case of a declared or suspected emergency, they will give full priority to the aircraft in need and will try everything to get the necessary support. However, depending on the type of aircraft, it might not be possible to find a pilot or another expert within the necessary time frame to do a safe talk down – and even with a talk down, a safe outcome would never be guaranteed.
ATC should be trained to get you the help you need if you're a passenger piloting a plane with a disabled pilot. They can get a pilot to come up in the tower, or get one on the phone.
There are probably many pilots on the frequency that would be happy to help. Also, it would probably be helpful to have a "wingman" who can tell you things like whether you are too high or low, fast or slow, directly.
They would certainly try but it's not part of their standard training. The reason is simply that events where both (or even more pilots depending on the aircraft model) are incapacitated are extremely rare.
In addition, air traffic control is not responsible for flying the aircraft, that remains the pilots' task for the time being.
