Batter's Box Question

[Fan10]

I was watching the Braves/Marlins game tonight, and early in the game, one of the Marlins players laid down a bunt. As his bat made contact with the ball, his right foot was entirely inside the batter's box, and his left foot was partially inside the batter's box, and the back part of his left foot was between the batter's box and the catcher.

As they showed the replay, the TV commentators started saying that he should have been called out. I didn't think that was right because I thought that as long as part of the foot was inside the box, it was okay. Am I correct there, or is the TV guy right?

Thanks in advance.