Thanks for the query.
There is less possibility for pregnancy
in case of regular periods, as her fertile period got completed by the time you had unprotected intercourse
The delay in periods could be due to the effect of the emergency pill she took.
For more details:http://srsree.blogspot.in/2012/08/emergency-contraceptive-pill-short-term.html
She can go for pregnancy test once to rule out the possibility.
If the test comes negative she can take tablets for withdrawal bleeding after consulting gynecologist.