Its the data use I suspect. She should turn off her data connection and only turn it on when she actually want to use it, also turn off push notifications (allowing apps to send you stuff) yes, she should check if there are any limitations on her free minutes, eg calling landlines or specified dial codes. It might also help to check her data usage balance, keep a note of what she does for a day, and then check it again. The other data usage thing I got caught out on once was allowing the phone to use geo location...lots of apps use it quite legitimately but if you keep it turned on it eats data. Of course all downloading of songs, apps, updates and so on should be done when connected to free wifi, at home is usually easiest.