HI, the likelihood that she is pregnant is slim unless she was ovulating, or near to it. Assuming a 28 day cycle, most girls will ovulate 2 weeks before their periods. When was her last one?
Vitamin C in large doses can work as an abortificant. If she takes a couple of grams every few hours, assuming there is a fertilized egg knocking around in her uterus, it may prevent the egg from attaching to the uterine wall and growing into a fetus. But don't rely on a home remedy, get her the morning-after pill if you possibly can. It's effective for up to 72 hours after intercourse.
Good luck. Hope she isn't pregnant.