What are the rules if a player has had his shot and the ball is left over the pocket then the oposite player goes to take his shot and the ball goes in the pocket before the player has had his shot?

The rules are as follows:

Player in control. A player is said to be in control of the table from the time that his body, cue or clothing touches the table prior to his shot, through the visit and up until his opponent does likewise prior to his visit. Any ball which falls into the pockets during this period will be deemed to have been potted. The player in control will be liable to any penalties or benefits normally awarded.

Hope this helps