Imagine you’re playing the 15th at Augusta. Most of us have dreamed of that in the past. You’ve gone through the green and face a devilishly difficult chip back towards the flag with the water lurking on the other side of the sloping green. Your playing partner’s ball is six feet past the hole as you're playing, almost directly on your line. It would be rather a nice safety cushion to leave it in place, to act as a potential backstop if your pitch is slightly heavy handed. It could prevent you going in the water! The question though is, are you allowed to leave it there?
Rule 15.3a says that if a player reasonably believes that a ball on the putting green might help anyone’s play (such as by serving as a possible backstop near the hole), the player may: Mark the spot of the ball and lift it if it is their own ball, or if the ball belongs to another player, require the other player to mark the spot and lift the ball.
