This brings up an interesting point, and hopefully it isn't lost buried at the bottom of this thread, but what about the implied collusion when two people both notice someone sitting out. It is not unheard of for people to take a "gentlemanly" though I'd say its actually, and aside from the tax reasons I hate using sport, unsportsmanlike approach of alternating blind stealing. Is this a form of soft play? Which is a form of collusion. Even if nothing is ever said between the players.
And on the point of soft play, and referencing the instance where the guy told his buddy to back off, had they been heads up and he simply not bet/raised that could easily be considered soft play and thereby a form of collusion. If I'm not mistake there was a WSOP
incident this year where a player was penalized for not betting the nuts when they were last to act.