This is one of the drawbacks to the points system, and I'm not sure I see a good solution. The guy with a won game is then in a position to decide, at no cost to himself, who is in second. Whoever he puts in third is going to get mad at him, either for cutting the game short or for dragging it on.
In other multiplayer games, such as Diplomacy, you're expected to pursue your own self-interest as you perceive it; that may help or hurt other players through no fault of their own, but you're just trying to further your own cause. Sometimes you might misunderstand your own self-interest due to tactical oversights, deception from other players, or unforeseen events outside your control, but if you're following your own self-interest as you understand it at the time, nobody can make any plausible claim of any violation of the spirit of the game.
That doesn't help us here. The guy in first has a won game, and neither helps nor harms himself by cutting off the game immediately or playing on to the death. His actions, then, are essentially random - and have a significant positive impact on one player and a negative impact on the other player.
I don't see any strong grounds for ethical guidelines either way. But it would be nice if we could come up with a way to tweak the rules to avoid situations where someone who doesn't have a dog in the fight can make a decision that has such a significant impact on others. I just can't think of a solution right now...
Often the situation arises that two players are left and the 2nd (ranked 3rd at that moment) player can't catch up to the 1st player (me in this case) by a long shot. The 3rd ranked player however, is ranked a few points below the currently 2nd ranked on points. I just kept on playing for a bit to so that the 3rd ranked player would catch up. I could have claimed a win, but thought the other player should be rewarded for his fighting. It was nothing like teaming during the game, but once we were the only players left, I allowed the other player to catch up to 2nd by playing until this player reached the necessary points.
Would you (and the team) consider this acceptable? I got some pretty ugly threats following the game by PM, so I want this clarified.