You co-worker is correct! Waxing your car will improve mileage. Any act which improves the coefficent of drag will increase the mileage.
HOWEVER, the real question is; HOW MUCH will waxing your car increase mileage....and that is where the rub is. The answer: .000001 mpg.
In chemistry we learned that adding a single drop will increase the level of water in the ocean...but not by very much.
My friend is an airline pilot. He says that every 3-4 flights overseas, the will have the plane washed. A clean 747 will save $20,000 in fuel from Detroit to Toyko. Now, that's a 747 and that's a long way. So, when taking a look at your tiny car and the tiny distance you drive, the practicle answer to your question is: Negligible improvement in MPG.