Clue by Clue: The Stiff Spy Solution
Natalya Hale poisoned the water that she used to make the ice cubes. She drank her drink while the poison was still frozen. Remember her second glass had no ice in it. However, after an hour or two in the hot sun, the ice melted. The poison entered the drink. Margrit had a second drink, because normally people drink whiskey slowly to enjoy it.