You defenitely should not sit your players.
How would you feel if you were the #5 team and were stonewalled out of the playoffs because another owner sat his players to help a friend? It is a classic "golden rule" situation. If you are looking for a philosophy that is a little more complicated, then Kant's philosophy of Universalism says that an action is only morally acceptable if it is acceptable that everyone be permitted to do it. If you don't think it is ok that all people sit players when it would help a friend (even when you are the one hurt by it), then you cannot allow yourself to do it.
Actions like this are also bad because they undermine the competitive nature of FF. If people will lay down to help friends (or make unfair trades, swap players to deal with byes, etc), then the game becomes less about how well you drafted and managed your league and more about how many friends you have in the league. In an environment where this is acceptable, people will approach the last few weeks not in an attempt to win the most games, but instead in an attempt to manpulate the standings in order to achieve the most favorable post-season scenario. There, popularity and personal interest overrule the competitive nature of the game and ruin it for everyone. In essence, such actions turn a FF playoff into the BCS.