Ball moved without being addressed

this happened yesterday and i wonder if anyone can tell me the correct ruling.

My playing partners tee shot stopped on a slope above the green. It took us a few minutes to reach the green and he then went to his ball. he was standing at his ball but he had not addressed it when the ball started to roll down the slope getting closer to the hole all the time. when it eventually stoped rolling it was about 3-4 yards closer to the hole than it had been.

Now I know there is no penalty because he had not addressed the ball but the question is should the ball be replaced to its original position or should it be played from where it finished?

If no outside agency moves it as you described then you play it from where it finshes, in this case a good result.