I'm not sure if is in any official rules, but I think one should be as nice as possible to players suddenly leaving the table. With that much money in the pot, it's quite possible that they've left for a medical emergency.
I think players leaving the table should follow these rules:
- Always check/fold EXCEPT
- When it's heads up (i.e. when only the missing player and one other player is not all in) and the other player can make a raise, then automatically fold (as the other player could trivially raise to force a fold anyway).
If it goes to showdown, I'd show the missing players cards, and if they win, it's their money. Keep it aside for when they come back.
I'd even go further, and allow players leaving the table to declare "all-in" out of turn, but only when there is a heads-up situation (i.e. only two players active not counting all in players). Acting before your turn with more than two players advantages some players and hence disadvantages others, and so shouldn't be allowed with 3 players, but I think with 2 it's ok.
My suggested rules perhaps are overly nice, but I think reasonable, particularly for a private game.