It is the night before our last session to finish Tomb of Annihilation.
For the previous three sessions we had seen things in the tomb that gave us a clue to what creature we might face. [spoilers]
…Our party saw eye stalks popping out of purple mold all over the Tomb. The party saw a door with 10 eyeball "keys." The party suspected for weeks that they were probably going to face a Beholder – just as described in Volo's Guide being sold in the Port of Nyanzaru. One player had once used Fog Cloud to shut down a Beholder in a different AL adventure just a few months earlier. He had cast it wrong so it didn't cover the whole area and the party almost died.
The player decided the next time he used it he would try to cast it right. This time, after it seemed clear what the party would face, the player decided he would again try the strategy of using Fog Cloud.
To prevent any rules confusion, the player posted this question on RPG StackExchange [spoiler link] so there would be a link to share if any debate or confusion came up.
The group had previously found RPG SE to be useful to quickly solve many rules confusions though occasionally looking things up slowed the game. For example, the party got confused about how a Fog Cloud canceled advantage and disadvantage. The player used a link from RPG SE to solve the confusion.
Then the adventure started. Sure enough the creature mentioned above was encountered. The player cast the Fog Cloud but it was dispelled for unclear reasons. The player thought perhaps an unusual object in the room was the cause but wondered if the DM might have had a different interpretation about how a particular skill of the creature interacted with that particular spell.
After the unusual object was destroyed, the player cast Fog Cloud again. This time, he said, "Now that object is gone – maybe this will work this time and it seems like it should work according to this" and he attached the link to the RPG SE question. [spoiler]
The DM asked, "did you just look that strategy up?" The player said no – it was an idea he had already had.
Later that night, the DM said that what that player who cast Fog Cloud did was "serious" metagaming equivalent to cheating and posted a link to wikipedia on the definition of metagaming.
To clarify, the Fog Cloud was an original idea of the player – not something found on the web. The player was accused of "cheating" for using their own idea in another game and for referring to a post they themselves created that showed their idea to prevent any rules confusion.
Per the DM, posting that rules clarification was cheating. We want to know if there is any support in AL for that interpretation.
The question is:
What guidance or criteria for AL exists for determining whether this is "serious" metagaming, and equivalent to cheating, for this specific situation of when a player uses the same strategy they used before on the same creature – but before using it posts the strategy on RPG.SE to ensure that there isn't a rules debate?
As this was AL play, the player could have argued his PCs knew each other. By a strict definition of metagaming, almost any use of a players brain using previous experience could qualify as metagaming. This question isn't focused on that binary, fine grain determination of "is" or "isn't" but rather how do we determine whether this is a "serious" metagaming that qualifies as cheating, violating the player-DM tenets of the game, and unacceptable in AL play.
Best Answer
First things first.
This is tagged as AL and I will be giving AL answers. First thing striking me is that half the post is about how the Code of Conduct was ignored essentially the entire session. From AL Player's Guide, p.6:
Just to make a point, the FAQ states these two important things:
Unless it's utterly clear that the DM has some rule wrong and completely prejudicial to the table (and if it was utterly clear, you wouldn't need to argue for dozens of minutes), all these discussions about rules shouldn't be happening, at least not during the session - note that even Sage Advice can be hand-waved by the DM. All this arguing is usually considered disruptive behaviour and the DM could simply kick the player out of the table.
Now about the Metagaming - is it cheating (according to AL)?
Metagame is not mentioned in the AL DM Guide. The Cheating section from the DM's Guide is pretty clear on what is cheating: things that are simply and straightforward against the rules of the game. Metagaming is not. The books don't say that your character doesn't know how to use Fog Cloud properly.
If you rolled a 1 and say you rolled a 20, you are cheating. If you are not tracking your spells and use 10 spells without a long rest, you are cheating. If you are not tracking your arrows and shoot 40 while having 20 in your bag, you are cheating. If you start your level 1 character with 16 stats on every attribute, you are cheating. If you use a spell that you know in a way that the spell is allowed to work, it is not cheating - the reasons you used that spell won't change that it is allowed by the rules of the game. You might be completely destroying the fun of the DM by doing that (and it seems you did :P), but that is not cheating.
On a note on that, nothing even stops you from reading the whole adventure and knowing each detail before playing and maximizing your plays - while some might call it bad sportsmanship or badwrongfun, if the whole table is fine with that, nothing in AL says you can't.
Stopping a game
AL is Session-based. AFAIK and from guidance, any player or DM can simply drop an adventure whenever they want - your character can play another adventure while one is already running (p. 14 of the FAQ), so that doesn't harm you in the sense of your character getting screwed. Nobody is - and shouldn't be - forced to keep playing a game/table that he doesn't want to play any more. If you think he DM's behaviour is prejudicial to the players, talk to the AL organizer. If there isn't an organizer, just stop playing with that DM, there isn't much more you can do.
The "I'm not sure about this part" part of the answer: you probably should be able to talk to the organizer and get another DM to finish the adventure. Either way, since ToA is a HC, you can also play the chapters individually - just ask/find another DM to run the chapters. Again, if there isn't an organizer, just find another DM the same way you found this one.
TL;DR
So, essentially, no, what you (or "the player", I'm still confused on that one) did is not "serious metagaming equivalent to cheating". Even if it was, the appropriate response for cheating is talking to the player and, worst case, kicking him out of the table, not punishing the whole table.
On the other hand, the DM, just like any other player, does not need a reason for stopping a game, other than "I am not having fun with it any more." - and he clearly wasn't. Even if the "metagaming" is completely acceptable, the incessant rules arguing is not.