In a tournament I was directing two players were playing in an intense time struggle, both with under 10 seconds left in a 30 minute game.
A player went to promote his pawn, grabbing the queen from the next board over. In the process of doing so he dropped the queen, with it falling underneath the table and then rolling away. I grabbed the queen for him, and handed it to him. If I had not done so he likely would have lost the game in running to grab it.
That player went on to win the game, and his opponent later complained about me aiding one player. Saying that I should have been impartial. I told the opponent to appeal if desired.
Was this correct? It seems foolish to not help, but in a sense I was directly helping one player win the game.
Having pieces available for promotion is allowed for a TD.
In this specific case the promoting player would have been entitled to pause the clock until an unavailable promotion piece could be provided by the TD, even if that also allows the player to analyze the position. Simply picking the queen up reduced the amount of reflection time the player might have otherwise gained due to a paused clock.
Actions that initially appear to be skewed in favor of one player are sometimes purely neutral actions that aid/penalize both players equally. If you always make sure the promotion pieces are available then any individual occurrence is an unbiased action.
It is a TD function to make sure that the desired piece for promotion is available. So you did exactly what you should have done. As Jeff mentioned the player was well within his rights to pause the clock and call for the correct piece. .
As long as the TD was already observing the game, a slight improvement would have been to make the replacement Queen a bit more accessible than the adjacent board. Otherwise, the TD action was perfectly proper.