I'm not completely sure what your exact question is, but this may help clarify things for you.
Keep in mind that an Elo rating only makes sense within a given population of players, and the difference between the ratings of two players serves to predict the result of a competition between those two players. Also, remember that the ratings of your friend's opponents are not static - just as your friend's rating changes over time as a result of his play, so do his opponents' ratings. There's no such thing as a "2000 player", per se - there are only players who are rated 2000 as a result of play within a given population of players. The Elo rating isn't like a "skill level" or anything, like, "if you have this rating, then you have X, Y, and Z skills".
When your friend plays an opponent, the difference between their ratings is a predictor of the eventual result. So if your friend is rated 1600, and plays 6 games against an opponent also rated 1600, then your friend's expected result would be 3.0 - which could come as 3 wins and 3 losses, all 6 games drawn, or some other combination. If the result is different than that - say, your friend scores 4.0, a better than expected result - then as far as the rating system is concerned, either your friend's rating is too low, or his opponent's rating is too high, or both. So the rating system adjusts both players' ratings - your friend's rating goes up, and his opponent's rating goes down. The amount by which the players' ratings change depends on how different the actual result is from the expected result - and the expected result depends on the difference between your friend's rating, and his opponent's rating. If your friend does better than expected against someone whose rating isn't too different than his, then his rating will go up by a little bit, by a small portion of the K value (and the opponent's rating will go down a little bit, by a small portion of the K value). If your friend does better than expected against someone whose rating is much higher, than your friend's rating will go up by a much larger portion of the K value. Depending on exactly what rating system you're using, and the ratings of the players involved, the K value for each player might be different, so your friend may not gain the same number of rating points as his opponent loses. And of course, if your friend does worse than expected against an opponent, he'll lose some number of rating points, and his opponent will gain rating points, depending of the K value, and exactly how different the actual result is from what the rating system would predict.
But, in either case, the ratings of your friend's opponents are not some fixed, static thing - they're changing too, as a result of actual play.
Does that help answer your question?