co-pilot incapacitation: land alone or with a passenger-pilot?
There are risks either case. There are no hard rules for or against calling a pilot from the cabin for help; it is up to the pilot to evaluate the situation and select the best course of action.

The risk of landing solo is greatly increased workload, which increases the chances of human error. There are quite a few things to attend to when flying an airliner (compared to a small aircraft). It gets worse when you're in busy airspace.

The risk of asking someone to help is the uncertainly of whether this person will do more harm than good. There would be differences in flight experience and operation procedures. However, Crew Resource Management and handling ATC radio applies to pretty much all types of flying.

Historically there had been examples of either choice, and they both turned out fine.