Set is a different story. I am talking by the game and you lose the first game and quit. You said he never loses more than 1 bet, so I assumed that applied in by the game scenarios too. Sure you can quit after a set. After a single game though, you’re bad action. That’s all. And as far as quitting winner, that’s not just bad action but it’s basically forbidden unless you 1. Play the person a lot and know you’ll get a shot at your money back. 2. Negotiate a stop time beforehand. 3. Up the bet and give them a chance to at least break evenSo how many sets do they need to lose before it's ok for them to quit? In my book, the loser can always quit whenever then want without any criticism whatsoever. ESPECIALLY when giving a huge handicap. If someone wants to get a good game and win more, they should ask to play for more. Sets are played for a reason; they're starting and stopping points.
Again, I'm from the old school and quitting winners is poor form. I have never heard of anyone getting angry at the loser quitting. Disappointed maybe, but not angry.