Should She Be Concerned About Delayed Periods Despite Taking Postinor-2 After Unprotected Sex?

Good morning sir/ma......a friend of mine saw her period on the 15th of September and she had sex on the 28th of September and she took postinor 2 after having the sex........up till now she has not seen her period ..........pls help her..........and today is 28days of not seeing her period.............pls help us....