Sunflower seeds — Food safety facts
Feed with care · Cats · full breakdown on the linked food page
Shelled, unsalted seeds in tiny amounts are fine. The shells are sharp and indigestible.
Short answer
A few plain shelled sunflower seeds are fine for most cats. Watch her closely over the next few hours for vomiting or diarrhea, which would mean she ate too many or picked up shells. Keep the sharp indigestible shells away and offer only a tiny number of plain seeds as an occasional treat. Call your own vet if anything seems off.