Here's the scenario. Friendly match with a few bucks on the line. Somebody hits his approach 18" from the pin. His opponent walks up and, before he can give the putt, his partner chips from off the green and hits the ball. Both balls carom to the apron about 12 feet away. The guy who hit it close says that he is going to replace his ball and his opponent (whose partner hit the ball) says he was giving it to him anyway and it wasn't necessary. The guy goes to pick up the ball and, while cleaning it, realizes that he picked up his opponents ball. Friendly match, so they replace it as best they could and carried on. The discussion at the 19th centered around what would have been the ruling if it was a real match play competition.
We asked a guy who is a rules official for the GAP, and he was stumped and said he had to think about it. My call is that he conceded his opponent's putt by picking the ball up.