On Lufthansa’s flight LH403 on November 18, the Co-Pilot on the flight from Newark to Frankfurt fell ill with a serious migraine which left the aircraft to be piloted by only the captain. Although it is perfectly safe to have only one pilot fly the aircraft, having someone in the cockpit to assist no doubt helps, especially should something happen to the remaining pilot.
Everything worked out well, as a passenger who happens to be a 767 pilot (who is also certified for the 747) for a US based carrier was on board and offered to assist the Captain.
Due to the nature of the medical emergency, the flight diverted safely to Dublin, Ireland (morning of November 19) to ensure that the co-pilot received immediate medical assistance. After a delay of a few hours, the flight continued on to Frankfurt.
This Journal.ie article contains a voice transcript of the flight as it approached Dublin.
source: DailyMail UK