How many strokes?

This guy has his ball marked about five feet from the hole .I'm about ten feet away and the other two guys are not even on the green yet.Well as I'm marking my ball I look up and see this guy stretching out and putting his ball while he is tending his own flag.The putt goes in with him holding the stick and his ballmarker still on the green,he then goes over and picks up his marker.I konw he was just trying to get a read on the putt.So I say you can't do that and when the other guys got on the green he felt guilty and now he remarked his ball.At least he missed the putt.The thing is it was a friendly three day tourament.How many penalty strokes should he have gotten or should we have just taken him behind the shed.By the way he won the tounament.Go figure.