Surprised to see this incident not brought up. Over Ireland, an Air Canada 767 F/O had a mental breakdown and became incapacitated. A flight attendant, who was a qualified CPL (but not in anything related to passenger jets), was asked to occupy the F/O's seat to help with checklists and radio work. The flight landed safely in Shannon.
Summary report in Cabin Crew News:
Cabin Crew News: Air Canada flight attendant praised after taking over for incapacitated co-pilot AAIU report - full detail
http://www.aaiu.ie/upload/general/11139-0.pdf
It's a good story, and good example of appropriate CRM - using the resources of the crew as fully as possible.