Thanks for the query.
According to the history there is less possibility for pregnancy
Delay in periods can occur due to many causes like stress
and strain, weight changes etc causing hormonal
disturbance, poly cystic ovary disease etc.
So let her wait for one more week.
If she didn't get her periods by that time let her go for pregnancy test once.
If the test comes negative, she can consult gynecologist once and take tablets for withdrawal bleeding.