How to monitor the network usage of specific apps on iOS?
You can do this with Onavo Count
Onavo Count monitors and analyzes all types of mobile data and phone use. This includes background, foreground, and Wi-Fi use.
iOS can do this natively under network usage. Just reset the stats with the bottom button and review later.
Go under Settings > Cellular and scroll down. You will see what each app installed on her device uses as far as data. (This is on iOS 7)