At both airlines I worked for in the U.S. it was written into the manual as SOP that in the event of 1 pilot in a 2-pilot crew being incapacitated, time permitting, an F/A would be placed in the jump seat, don a headset, and serve as an extra set of eyes, ears, and read checklists as instructed. The F/A is also there to help immobilize/calm the incapacitated pilot (if he can't be removed from the seat, or wakes up incoherent) and administer whatever first aid can be rendered, allowing the other pilot to concentrate on flying the airplane.