When screening a stationary opponent from the front or side (within the visual field), the screener may be anywhere short of contact. When screening a stationary opponent from behind (outside the visual field), the screener must allow the opponent one normal step backward without contact. In this case it appears the player setting the screen is still moving forward and impedes the defender. It should have been ruled an illegal screen. Call this early in the game to set the tone and clean up play. -Terry

Rule 4-40