To answer your question "It can't be deleted; can it?" -- The answer is Yes, it can be deleted. If the phone was not backed up then you won't be able to retrieve her data. If it was not backed up through iTunes, it was probably backed up through iCloud.
What is displaying on the screen at the moment?
Did you perform the restore back to her iCloud backup? It will restore everything that she had backed up last.
Yes, please perform the iCloud restore on the phone using your mom's Apple ID/password.
The option to do the restore to iCloud has to be done on the phone; not through the iCloud website.