Illegal Screen

[slickrick]

How much movement would you allow a player to moved another player while screening?

I was watching the Stanford / St. Joes. game yesterday and I was wondering about a certain play.
The game had less then two minutes left. A St Joes player was driving to the hoop and he was fouled in the act of shooting on a lay up. ( I think the score was tied ) As I got to watch the replay it showed the low post player (St Joe off ball) pushing the Stanford player back towards the free throw line clearing a path for his player. In which the Stanford player fouled the shooter.