The GM's mistake was to ask for a saving throw before every player had declared to eat a fruit or not.
If it's a tasteless poison, it's going to take a minute or two to kick in. (Probably even much longer than that.)
This is the best answer.
If the scene is supposed to be a trap, then the trap shouldn't spring until the end of the scene. It shouldn't spring until the PCs have realized that a possible danger actually is a real danger.
As it is, even if we assume the mistake wasn't a mistake and the poison was an immediate effect, I'd kind of like the game to shift away from being pejorative about this kind of "metagaming" play. I think I've really grown to dislike the style of play that the only communication between players should be "in-character" communication. I don't think that creates the most enjoyable experience, and it mostly encourages players to feel guilty about "metagaming". I increasingly think that it's entirely valid to metagame this way so long as you continue to roleplay your character.
I feel three things to be true:
Firstly, the players can and should collaborate and discuss with each other at any time what they think is going on and what they think the best courses of action should be.
Second, players should role-play their character, meaning each player should decide what their character does based on what motivates that character.
Third, the first truth has
nothing to do with the second truth. They're actually
totally independent of each other and neither is more important than the other. If we're going to trust that players should have their character behave in-character, then we should actually just do that. We already should expect that sometimes characters will do the opposite of what we as players believe or know to be true, so we should stop fighting against that.
Even if "your character isn't there" you're still a player at the table. You're still a player in the game. You're still responsible for verisimilitude and narrative just as much as the other players and the DM. There is no reason that you shouldn't have input in the collaborative story that the players are discussing in the game world. Indeed, this may be exactly how best to emulate characters being smarter than their players. Intra-player communication should always be open.
I'm sure some people will be urged to comment, "that's what we've done for 50 years and it's how you've always been supposed to run the game," but I really don't think it's how the culture or community or rules of D&D, in particular, started or have grown.