That is really the only difficult part.
Once you have a non-pilot that is coherent, not panicking, and that operated the radio well enough to actually talk to someone for assistance, the rest is not that difficult or dangerous.
Assuming a fully functional plane, and good weather. (lets not introduce anything that will cause real pilots distress…)
Also assume the plane has enough fuel for 2 hours+, allowing time for the controller and non-pilot to actually work through the problem.
Preventing the plane from nosediving into terrain: 98%
basically, this meant preventing the non-pilot from doing something abysmally stupid in the first 30 seconds.
Navigating to somewhere near a suitable landing field: 100%
Lining up with the runway, or more likely with a suitable flat uncluttered piece of ground: 100% (may take several attempts)
Getting aircraft ready to land: 99%
(stuff like flaps, wheels down, etc. Odds this good, because almost everything can be verified before needing it)
Not stalling on final approach, not nosediving before landing spot: 90%
Landing gently enough to not roll/skid/tumble: 60%
vs.
Surviving if landing is botched: 95% (yes, modern planes are quite safe, if you don’t stall and nosedive the ground)
So basically: Safe landing completely unharmed, about 50%
Unsafe but survivable landing, about 45%
stuff up and die, under 5%
Remember that this scenario requires things to be valid:
the plane in question must be a simple, single-pilot, preferably single engine light plane. No large airliner or equivalent!
non-pilot must not panic, freak out, or otherwise go nuts.
non-pilot must be able to use radio well enough to establish initial contact with controller.