Wow. It amazes me that things like this still happen. Apart from the obvious stupidity of a non-TRE PIC allowing seat swapping on a revenue flight, this shows that motor responses of the left and right sides of the brain are not equal.
The right side of the brain controls the left limbs of the body, and vice versa. Swapping seats means swapping the sides of the brain in control of the thrust levers and the yoke. Also, the view of the landing from the other seat will have the various visual cues during the flare flipped left to right.
Any motor control within in the brain requires practice and repetition to develop and produce the fine hand-eye coordination required with flight control. Just try writing with your other hand to show that.
Allowing an untrained and unpractised person to swap seats in flight and then attempt the fine control needed to land is such utter stupidity.