The rating system rewards won games. Not just played games. But rating also depends on at what rating you initially started at the site.
Actually the rating try to be a measure of your playing strength, and by that predict your results. So if you and your friend played a rated game, then the system will predict your friend as favorite. If you won or draw then your rating will increase (you have performed better than predicted) and your friends rating will decrease. If your friend won, then his rating will increase (he has done better than predicted because he was not a 100% favorite, and your rating will decrease (you should in the long run score more than 0.
How did it happen? I've been playing on the site for a year now (I started playing with 500 elo), I played 320 games to get 1000 elo, my friend who registered this year played only 14 games and his highest rating is 1200...