My Fiancée Took Postinor 2 On 8th December 2016. Her

My fiancée took Postinor 2 on 8th December 2016. Her menses was expected to come on 23rd December 2016, but up till today Sunday 8th January 2017 she hasn't yet seen her menses. This is the first time she's used Postinor 2, and this is also the first time her menses is delaying for that long. Please, is this delay normal? What could be the reason for it? And what's the possible remedy to this? Thank you.