I am shocked the employee would try this, as it could jeopardize his flight benefits if the gate agent reported him.

Honestly, though, with the way people dress on planes nowadays it would seem silly for UA to continue a dress code. I have the information somewhere around here, but I think it goes something like this:

No jeans, no shorts, no tee-shirts.
Must wear leather shoes.
No sneakers.
No flip-flops.