Someone could cain access to her personal info, passwords etc. and use that info elsewhere. You do not want hackers accessing any of yout information!
It sounds as though something went wrong with the upgrade. As I said I have been using iOS 12 for around 6 months at least and I haven't found any issues using it, either here or for any other function.
I only upgraded my iPad to iOS 12 recently and the biggest issue I have with that is the keyboard layout — but I'll get used to it.