It seems that Elsa smiles after Donovan drinks from the false grail implying that she knew it wasn't the right one.
Did she betray him?
If we look at the script and the film's official novelisation (itself based on the script), we can see that Elsa clearly set him up to fail.
DONOVAN: I'm not a historian. I have no idea what it looks like. Which one is it?
ELSA: Let me choose.
DONOVAN: Thank you, Doctor.
Elsa and Indy exchange looks. He thinks he is seeing her in her true colors.
Elsa chooses a cup -- a solid gold, emerald encrusted goblet.
Donovan instantly takes it from her.
DONOVAN: Oh, yes. It's more beautiful than I'd ever imagined. This certainly is the cup of the King of Kings.
Elsa removed her hat and carefully picked up a shiny cup encrusted with sparkling colored stones. Donovan instantly grabbed it from her and held it up to the light. “Oh, yes. It’s more beautiful than I had ever imagined. And it’s mine.”
Indy expected Elsa to protest, but she remained silent. The knight’s face was implacable, revealing nothing.
Since we already know that she's a competent grail scholar, her mistake was unforgiveable.
My first initial thought was that she had given him the wrong grail on purpose, but then at the same time if she had, why did she seem so surprised and horrified that he was killed by drinking from the grail she picked out? The knight stated very clearly that the wrong grail would "take it(life) from you." I'm sure it was on purpose but still, just a thought, perhaps she was just freaked out by HOW the wrong one kills you.