No - if anything, you should be buying her a new copy of her game, explaining that you can't find her old copy, and then trade back. If you find the game again later on, then you've only gained a game out of it.

And if you don't find the game again, it's not good enough to say to someone "Oh, you can just keep the FFVIII I gave you instead." They should get the game back that they gave you. *nod*