She had an iCloud backup, which had several hundreds of pictures in the camera roll.
The problem resides in the passage from the iOS 5 backup to the iOS 6 environment of the new iPhone, which came with the newest iOS installed. Ther are some issues when a large amount of pictures need to be restored in the camera roll.
The main problem is that the camera app will not save any photo in the camera roll, so all the pictures will be lost (fortunately there's a trick to recover them).
The photos app will show just the pictures restored from the iCloud backup.
This is what you need to do in order to solve this issue, without having to completely restore the iPhone as a new iPhone:
Download Phoneview, or iExplorer, two handy programs which are designed to get into the memory of the iPhone and backup specific files or folders.
Since most of you have non jailbroken iPhones, the up most directory available will be var/mobile/media. Go to the DCIM folder, and backup all the folders inside. They contain the pictures that were in the iCloud backup. The ".MISC" folder will contain the pictures that were not showing in the photos app.
Ok, now that your pictures are safe, in your iPhone settings go to icloud, then backup (or archive, I don't know the exact translation), then manage backup.
Click on the backup where it says "This iPhone", in the backup options deselect the camera roll, and keep activated all the other variables. Go back, again go back, then push the button "backup now".
This procedure starts the creation of a new backup, this time without the pictures that are now in the iPhone. This is done because these pictures cannot be manually removed by any mean other than restoring from a modified backup: neither by using an external software as Phoneview, nor by any internal command in the iPhone. Deleting the pictures from the camera app, will remove their listing from the camera app database, but the file will still be in the DCIM folder.
After the successful creation of the new backup (annotate the time of its completion), restore the iphone by going in the general settings, then reset, then click on "Erase all content and Settings". This will be followed by two security confirms.
The iPhone will reboot.
As it restarts, it will ask if you want to restore from a previous iCloud backup. Say yes and select the backup you did just a couple of minutes before.
Your iphone will download the backup, then restart. After some minutes, you will see the typical dashboard, and the system will tip you that the iPhone was restored from a previous backup.
It will then start redownloading all the apps that were installed before.
The photos app will show no pictures, besides the photostream, and the camera app will save any photos in the camera roll, just as usual! Problem solved!