Thanks everyone for participating again this week, we appreciate all the good discussion!

Bernie Hoelschen was quick to the right answer. Allie has missed her trigger, and it is generally detrimental, so we step in and issue a warning.

MIPG 2.1 - GPE - Missed Trigger (Definition)
Judges do not intervene in a missed trigger situation unless they intend to issue a Warning…
(Additional Remedy)
Upgrade: If the triggered ability is usually considered detrimental for the controlling player the
penalty is a Warning. The current game state is not a factor in determining this…

Allie has committed a Game Play Error – Missed Trigger infraction for not remembering to sacrifice her Labyrinth Guardian before letting Nemo’s Final Reward resolve. Since Labyrinth Guardian’s triggered ability is considered generally detrimental, Allie will receive a warning for the infraction. Pause the game and indicate the missed trigger to the players. Nemo may choose to put the trigger on the stack now, but it won't do anything as it resolves.

(Additional Remedy)
If the triggered ability created an effect whose duration has already expired, instruct the players to continue playing.

It doesn’t matter that her missing this particular trigger ended up being a worse outcome for Allie than if she had remembered it. A detrimental trigger is always considered detrimental regardless of the current game state. Nemo did not do anything illegal in this situation because he did not control the triggered ability that was missed and therefore is allowed to ignore it for his own benefit.

(Philosophy)
Opponents are not required to point out triggered abilities that they do not control…

It is also worth drawing a parallel of this situation to a missed Chalice of the Void trigger. There will often be times where our prescribed fixes are too late to clean up what they were supposed to, but we don't need to engineer some alternate fix because what is in the IPG isn't perfect.