I may or may not agree with that LOL. It would be similar to someone walking up to the table as you are shooting but you shoot anyway and miss. Yes you missed, but the game was over when the other player got up. In this case the shot was hit but is not over till the cueball stops rolling. Now I am in a grey area. The shot was started when the player was sitting so not conceding, then the player got up to shake hands which IS conceding the game. Ah... you got me in a spot here LOL.
I may need to make a ruling based on the skill and overall knowledge of the player. If the opponent is a newer player or only plays in leagues in bars, with more limited knowledge of sportsmanship rules, I am likely to make the call for the opponent, meaning he just made an honest move to say nice job without any intent to concede the game. Since that happened after the shot was made.
If the sitting player is a seasoned vet of tournaments, plays with pros in Joss tour, etc.., I may rule that they knew shaking hands before the game was over was a concession and rule the game over then even if the shot was not. They should know better if the cueball is running around the table than to call it a loss or a win till it stops.
Hopefully that logic makes sense when read like it did in my head LOL.
I think it's a problem when someone starts to put away their cue before the last ball is down. I'd consider that a concession. But there's no sharking here. There's no incentive to pretend like you're conceding the game. But then again, this is the league that requires marking the pocket...
This reminds me of an issue that came up once - one guy tried to claim that, since the 8-ball dropped in the pocket BEFORE the cue ball scratched, he won the game. lol he didn't get away with that.