How to downgrade from iOS 7 to iOS 6 on an iPhone 4S?

I updated my sister's iPhone 4S by downloading the torrent for iOS 7.To download this, I used the "Check for Update" button, pressing "shift" and right clicking on it. From there I selected the iOS 7 file and it started installing to the phone. However, during the downloading process, an error occurred. The only option was to restore the phone, so I did. The phone restored with iOS 7 software and will now not allow me to do anything with it. After trying to access the phone, a screen shows up saying you must be registered as part of the iPhone Developer Program. My question is, is there a way to delete the iOS 7 software and replace it with the iOS 6 that was formerly on the phone. (If at all helpful, I can't gain access to the page that allows you to look for updates/restore the phone again)

Unless you've saved SHSH blobs don't bother trying. Apple has made it 100% impossible to downgrade iOS devices that do not have previously saved SHSH blobs.

However you may try to take it to an Apple store. They may help you in this situation as you were not trying to jailbreak, etc. But if shift-restore doesn't work there isn't much you can do single-handedly on your own.