Continuity mistake: At the snake exhibit, while Harry speaks with the Python the snake's body lies curled over a long wood log and a large rock. In the shot from outside the Python's exhibit, after the glass pane disappears Dudley falls over the rail and the snake is gone, not positioned differently or blocked from view, but altogether gone. In the next overhead shot inside the Python's exhibit, while Dudley falls the snake is back lying on the log and rock, in the very same position as in the previous shots. (00:07:10)
Answer: Petunia or Vernon would have ran to a zoo employee and told them what happened. The employee would then open the door to the snake exhibit letting Dudley out.