My opponent had Nisaba copying one of my science cards (the feather, but it doesn't matter which). He didn't have his own copy of the card. Later, he activated the conspiracy which swaps blue or green cards with your opponent. He took the science card which had Nisaba on it and swapped it for some other irrelevant card. I assume he was hoping that this would give him 2 of the same science symbol: the card itself, and Nisaba targeting it. However, the game ruled otherwise: the Nisaba image fell off when he took the science card, leaving him without a match. Is this correct?
From the wording on Nisaba, it's a little unclear. It does say that you put it on an "Opponent's card" but that doesn't have to mean that the opponent needs to continue to control the card for Nisaba to have its effect. Nisaba says that "Nisaba" becomes the symbol on the opponent's card, possibly implying that the little token on opponent's card is just to remember what symbol it was, and that once you play Nisaba it no longer matters where the original card is. In our case, perhaps Nisaba, upon playing, becomes a feather symbol for the remainder of the game.
From the wording on Nisaba, it's a little unclear. It does say that you put it on an "Opponent's card" but that doesn't have to mean that the opponent needs to continue to control the card for Nisaba to have its effect. Nisaba says that "Nisaba" becomes the symbol on the opponent's card, possibly implying that the little token on opponent's card is just to remember what symbol it was, and that once you play Nisaba it no longer matters where the original card is. In our case, perhaps Nisaba, upon playing, becomes a feather symbol for the remainder of the game.