That is normally a feature which can be configured within the phone itself. There should be an option where you can set a data limit which is reset on a specific date, which you set to the billing date.
She should be able to monitor the current mobile data usage, and set an alert when the limit is reached, or is close to being reached.
Oh ok. I'll take a look. Thanks
Its certainly a feature on Android phones, and I would expect it to also be available on Apple phones.
Android phones - settings - data usage - Cellular data. Options to set billing cycle date and warning level.