Hi there @Kbl1
Thank you so much for your first post to our community forums and welcome on board, it's great to have you on the team.
I'm so sorry to hear this has happened with your Mum's phone, it does seem odd that the data would be draining so suddenly. Can I ask if you have been able to check the data usage in the settings of the phone? You can usually see which apps etc are using the most data.
Thank you.