If the plane is not filled, and if the seat arms can be folded up, passengers to the left and/or right of the affected person may be reassigned to other seats, so that the person having the seizure can be helped to lie across two or more seats with head and body turned on one side.
Once consciousness has fully returned, the person can be helped into a resting position in a single reclining seat.
If there are no empty seats, the seat in which the person is sitting can be reclined, and, once the rigidity phase has passed, he can be turned gently while in the seat so that he is leaning towards one side.
Pillows or blankets can be arranged so that the head doesn't hit unpadded areas of the plane. However, care should be taken that the angle at which the person is sitting is such that his airway stays clear and breathing is unobstructed.