Ball moved on Green

I have just got back onto the course this last week and came across a situation i am unsure about.

A playing partner marked, lifted, cleaned and replaced his ball on the putting surface and took his marker away and went and to take a line for his putt when the ball moved and rolled down the slope on the green.

What is the ruling,

I thought at the time that he has to play the ball where it now lies and could not replace his ball.

 had he addressed his ball i believe he would have incurred a 1 stroke penalty, and again plays the ball as it lies.

What constitutes addressing the ball on the putting surface?

Any one with any advice welcome

Grounding your putter behind the ball is deemed addressing the ball.

I think (not 100%) in this case he could have replaced the ball. 

I thought that once the marker has been lifted if the ball moves it is played where it lies, unless the ball was adressed with the putter or the player was deemed to have caused the ball to move.

addressing the ball is placing the putter behind the ball and even if you move the club away you have already adressed the ball.

This was interesting because on a recent tour event it was very windy and none of the players dare place the putter behind the ball for fear of it osilating or moving in the wind.

Colin,

If a ball at rest moves under its own steam it is played from where it ends up. Sometimes it is better for the player, sometimes not.

The act of Addressing a Ball is complicated. The Rules state that you need to "take your normal stance and ground the club". What it doesn't say is what actually is "grounding". As some have already said, is it placing the club on the ground behind the ball? Sorry but most Tour Referees would still penalise you if you just placed the club on the ground anywhere.

I am a member of 'The Leith Society' Forum, a forum for Rules gurus and this had a very long debate with no real correct answer for what actually is "grounding a club".