I would like to know if you can use the defibrillator on a sitting person, or if that person has to lie flat.
There are no restrictions that I know of against using a defibrillator on somebody in the upright position, however with somebody in cardiac arrest CPR needs to be performed as the AED warms up and analyzes the rhythm before it can be activated. CPR then needs to be continued if the shock isn't successful until the next shock can be administered. CPR is only really effective with the patient lying down on a hard surface, so that would require moving them from the seat to the floor.