It could well be data usage (planned or unplanned). disable mobile data on the device and her wifi should still work. What device has she? If she wants to use data then look at the PAYG plans and choose one with data.
Cookie Vodafone Community Expert (or so I'm told) and S9 owner. I don't work for Vodafone
Sorry for the delay in our reply. You can ask her to register for a MyVodafone account through the Vodafone website. Within the account, she can see a breakdown of her usage.
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