"My skin was cooking," said Vouncile Lambert, 44, who was treated at a hospital.
The 17-year-old worker was charged with aggravated assault. The employee, who had worked at the Checkers restaurant in West Philadelphia for about two months, will be fired, a restaurant manager said Wednesday.
Lambert's 36-year-old niece had spit on the employee, a police report said.
Shaji Joseph, an area manager for the hamburger chain, said the customers were upset when the employee closed the walk-up window on them Sunday afternoon.
"Nobody would just throw grease at somebody without provoking," Joseph said. "(But) we totally understand she can't throw grease on her."