I know unexpected charges can be quite inconvenient. May I know where you're viewing the break down of what the data was used for? Is the usage for this month significantly higher than her typical usage?
Depending on the plan type, there are notifications sent out when you're close to using up the data included in your plan.
If you'd like us to review the usage and billing in detail please reach out to us via PM @CommunityHelps so we can assist you further. If you're not familiar with our private message system, please check out this blog.