Earlier quoted context omitted.
Well, we do not know what would happen if the situation were opposite. They both entered into the agreement, presumably seriously. It seems like just playing the game qualifies both sides as jerks.
If the roles were reversed, I would have a similar criticism of the other guy. Clearly, the loser didn’t want to pay because he contested it in court — and the victor could have forgiven the debt. I wouldn’t even make my friend pay me back for lunch if they forgot their wallet — let alone put them through financial hardship for such a trivial, low-effort event as a game of RPS.
That's a bit different. Your friend is presumably not deliberately forgetting his wallet while eating $500k lunches, and if he was I bet you'd expect him to pay you back. And the game of Rock Paper Scissors may be trivial and low-effort, but betting $500k on the game is very much not.
If I enter into a $500k bet on a game of Rock Paper Scissors, it's with the understanding that I stand to lose $500k if I lose the game, and I take that seriously. The only reason that risk is worth taking is if I also stand to win $500k if I win the game. If I win and you turn around and say "oh come on man I really don't want to pay", then you're basically saying you entered into the bet with the intention of scamming me out of an expected sum of $250k, so yes I'll insist that you pay up.