If this is a yahoo league, then I say tough luck. I'll explain. If it were a situation where he clicked on the wrong name by mistake, ok, I say let him have him back. But this can not be the case here. In yahoo leagues, when you pick up a player, they list the possible players to be dropped alphabetically by last name. So, basically, Matt Anderson was listed at the very top, and Mike Williams was listed at the very bottom. So therefore, he didn't
Now, if it's the case where he thought Mike Williams was the Detroit pitcher...then tough luck. If you don't know the players on your own team well enough to know the difference between Matt Anderson and Mike Williams, then you should pack up your bags, and head to the minors.
The only way this guy is going to learn his lesson, is by getting screwed. Maybe then he'll learn to double check his transactions before he signs off on them.