Just in case you change your mind:
If there are apps on her iPhone that she doesn't use, delete them from her device to free up some space. If she needs them again, she'll be able to upload them again from the App Store (Purchased tab).
If she has many photos in the Photos app, it's a good idea to save them somewhere else. Online Storage apps like Box, Dropbox, Google Drive, OneDrive and others are able to upload them automatically, and then she can remove them from the Photo app, which will also free storage space on her device.
In case she opened attachments in the Mail app, this will also add to it's storage space. Delete the account from her iPhone and set it up again, this will reduce the amount of storage for the account.
These are just a few things you can do to get more storage and update her iPhone. And in her case, it's better to use iTunes (computer) to update it.