Well, give it a couple weeks after the date she expects to get her period, to be on the safe side.
One thing - and not to be negative, but just realistic - sometimes a "late" period is actually a miscarriage. That happens surprisingly often.
So - be patient. If there's no period for two weeks after her expected date, then you could pick up a home pregnancy kit from the drugstore. Doc visit to follow if it's positive.
I wish you luck!