The passenger wasn't struck by lightning but just having the electrical current of the bolt passing through the skin of the plane a few inches away from her could have done a number on her pacemaker.

Planes get struck by lightning all the time (usually when they are in the air) and a strike is usually a non-event. Damage is unlikely but not unheard of and since they hadn't taken off yet and had to return to the gate anyway, I guess better safe than sorry.