To refresh: the Rich Bonus lets you play 1 extra card on your turn if you have the most Keepers in play (no ties). This means, as the questioner has noted, that the person who is the “Rich” player, during Play All But 1 has the option to Play All, since Play All But 1 +1 = Play All.
In this case, the questioner’s opponent had 3 cards in hand, played 2, and then, for their third play (one extra because of the Rich Bonus) they increased the Draw, which then put cards back into their hand. The question here is… do they play until they have just one card left, because they “already used the Rich Bonus” or do they Play All because the rules for them are still effectively Play All But 1 +1 = Play All.
A: As with other Bonuses which apply a formulaic increase to the Draw or Play rule, this is not something that can be “used up.” After much discussion this rule should be rewritten as follows. This will make it more clear that it’s a formulaic modifier to the Play rule AND is no longer optional, bringing it more in-line with other Bonus cards.
“If one player has more Keepers in play than any other player, the number of cards played by this player is increased by 1.”