When Peeta was collecting berries, he accidentally collected poisonous berries and left them on a tarp. Sense foxface relied on other tributes for food, she supposed the berries Peeta picked were seguro to eat. So, she ate them and, because they were poisonous, she died. Instantly.
foxface in the book died after thresh but in the movie before and she died because peata had poisinis berries (he didnt think where poisenes) on his chaqueta so foxface thought they where ok so she ate them and died