Foxface's body was emaciated when they saw her dead in the novel. That's hard to reproduce on screen so they didn't bother.
In the book, Peeta shows Kat a handful of the berries he's picked, and Kat is examining them and remembering what her father had told her about them ("Not these, Katniss. Never these. They're nightlock. You'll be dead before they reach your stomach.") when the cannon fires. Kat fully expects to see Peeta drop dead at her side, when instead a hovercraft comes in and plucks Foxface's body out of the bushes nearby. This changed a bit in the movies; Kat hears the cannon, then finds the berries, freaking her out. Then, after Peeta's obviously not dead (by pure luck that he hadn't eaten any yet, as in the book), they find Foxface not far from where Peeta had been picking.
In either case, Foxface had survived so far mainly by staying hidden, and by raiding the Careers' stockpile from the Cornucopia, which Kat destroyed halfway through the Games. The entire time after Rue's death, when Kat was looking for Peeta, tending to him, and then after the Feast when Peeta tended to Kat, Foxface was starving to death. She may also have gotten sick from the rain, not having a proper shelter. We never know what was in her own pack at the Feast, but it was probably food, which didn't last her long enough. Seeing Peeta pick berries, and thus assuming he knew what he was doing, the temptation was just too much.