While the city is only 15 minutes drive from the airport at times, given your narrow window, it would make more sense for your friend to come to the airport to meet you. Indeed, Wikitravel suggests that in busy periods, it can even take up to an hour, which you don't want if you're stressing about getting back to your flight!
Assuming your bags are checked through to your final destination, all you need to worry about then is yourself, carry-on and passport/visa situation.
For your carry-on and self, remember that you'll still need to go out through passport control, and get back in through customs, security and passport control. This is common enough for a few hours, I've done it in several airports where I had a longish layover - I wanted to check out the city.
Then your passport/visa - as long as you're just visiting (as you suggest) then Guatemalan citizens get a 180 day entry, no visa required. So it should be a case of zipping through passport control, meeting your friend, and have some time to enjoy the airport, making sure to allow yourself enough time to get back through to your plane.