Had he taken 5 minutes to show up to Autozone and gotten the code read out for free that's what would have popped up.
There are several good threads on this forum dealing with the issue. In most cases without other check engine codes popping up at the same time (which we don't know) the gas cap is at fault. There was one guy on the forum who "fixed" his gas cap by replacing the seal around the cap and making it air tight again for less than a dollar. But of course you can also just go and buy a new gas cap as long as it fits.
Hope that takes care of the problem for you!