if she's on the pill and has been for a while don't worry about it. if her period is late have her take an hpt but i wouldn't stress to much about it.
If she's on the pill too, I wouldn't be too worried, and even if she wasn't, as long as you grabbed it out quick I wouldn't worry. Her period coming early could be caused by stressing about the possibility of being pregnant, I highly doubt she is. Good luck! :]