Well if the phone tries to backup and there is not enough disk space on the PC hard drive....well, maybe there is not enough disk space. And as a Focus can theoretically have 40GB of storage (8GB Nand + 32GB microSD) that is entirely possible. Remove all media (photos, video & music) from the phone and try again. If it fails again, remove all the games, and the apps is necessary. She can reinstall everything again after the update.
Sent from my Lumia 900 using Board Express Pro