If there's only one human player left in a tournament game, that player should get the win. From the tournament page, this game should be considered "finished" even if there are turns left. The last player would then be free to play the AI, resign, or abandon.
This would also solve another issue which is that this last remaining player can hold up the tournament indefinitely by not loading that game. I haven't seen anyone do this intentionally but I've had to ask many times that people please finish that game so that the tournament can move forward.