morning after pill...
sometimes not available in convenience stores,
in the UK they are available without prescription from chemists with pharmecys, or from a doctor. (not certain about the US)
they are most effective if taken right away and still effcetive, (though gradually less effective) for 3 days after. -so take it sooner rather than later.
go to a chemist with her and get the pill to take care of it. (unless you really want a child).
the pill basically destroys the lining of the womb, so regardless of whether it's that time or not she'll likely have a very painful period and feel quite sick.
obviously if she is physically sick within a few hours of taking the pill she'll need to take another one.