When any opponent quits, the game ends. The guy who quit gets last. Fine. Good.
But this creates multiplayer problems! It looks to me like the non-quitting players are scored as if the game ended naturally: whoever has the most points (and didn't quit) wins. Either that or it's a tie between all non-quitting players; I can't actually tell, since the box just says "Opponent Quit" and puts that opponent last no matter his score.
Either is problematic. The game should simply continue without the quitting player.
In the game I just played, my opponents both played terribly. They went Counting House Big Money??? Anyway, it gave me PLENTY of time to set up my Madmen/Goons engine. Mwahaha. I executed it once, picked up 30 points, and then set up to execute it again. Well, AS I was executing it the second time, one of my opponents quit, and the game ended in the middle of my turn. (Which sucked for me, because I had just played 6 Goons! Think of the VP!). Thankfully, I still had the most points, but just barely.
So this is a problem.