[Takket]

Supposed I have Shades of Gray: Cruelty face up on the table (activated sometime earlier in the game).

My opponent red shirts some mission and hits Shaka. "Well I obviously don't have 2 diplomacy and cunning > 30, so I'm stopped."

They shuffle around their personnel, come up with 2 diplomacy and cunning > 30, and attempt again.

NOW, I point out, "Hey, i have SoG, you also need 2 exobiology

Am I under any obligation to have pointed that out the FIRST time they hit Shaka, as part of reading the dilemma, or are they S*** out of luck for forgetting I had it face up on the table from earlier in the game?

Less obvious question: Is it bad sportsmanship to not point out I have a card on the table that changes the text of the card they are reading?

[jadziadax8]

I think you needed to say something when they encountered the Shaka in the first place. The exo x2 is part of the dilemma requirements now.

[Iron Prime]

I don't think you are required to say anything, though @LORE would know for sure.

That said; I think it's probably bad sportsmanship unless you are at Worlds or something (maybe even then too)

[AllenGould]

I don't think you're required to say anything unless asked or it's relevant. You can't lie if they ask what the requirements are (or similar), and you can't conveniently also-forget about the additional requirements (say, in order to call a judge on them, or because you can see the next dilemma will mess them up).

[LORE]

Supposed I have Shades of Gray: Cruelty face up on the table (activated sometime earlier in the game).

Am I under any obligation to have pointed that out the FIRST time they hit Shaka, as part of reading the dilemma, or are they S*** out of luck for forgetting I had it face up on the table from earlier in the game?

Less obvious question: Is it bad sportsmanship to not point out I have a card on the table that changes the text of the card they are reading?

SoG:C (and similar cards) are all triggered off of certain dilemmas being encountered, so I would either state when it was flipped which dilemmas it effects or (my personal preference) hand them the card to read it themselves.

Provided that has happened, I agree with @jadziadax8 that the Exo is now a requirement of Shaka and when it is first encountered via the red shirt in your scenario you should tell them it is impacted by SoG:C. That said, you are under no obligation to allow a "take back" if they subsequently attempt it and forget about bringing the Exo.