Seriously?
Someone really returned an article of clothing because the "shadow was on the wrong side"?
Why does it have to be a shadow, why not a branch that was recently rained on for a brief moment, they are darker on the top. Or what about moss or a fungus growing on the top side to get light from the sun.
When an animal has the ability to know the meaning of a shadow, what it is and the correct orientation of it, we have way bigger problems than if they are going to spook because the shadow is on the "wrong" side.
I really hope you are on the common sense side of the arguement in the office.