How to restore iPhone from ios 7 download?

My boyfriend Downloaded ios 7 beta, updated when update popped up on my phone. (Not a developer). Phone started getting slow and crashing. He wanted to get rid of ios 7 beta. He download 6.13 to computer, on itunes clicked shift+ restore, opened file, downloaded to my phone. His phone was back to 6.13 but all of his files were missing and his phone said he need ios 7 to retrieve back up. Tried to go back to ios 7 by doing shift+ restore, once that happened its stayed at the activation screen saying "Activation Error" This is not registered as part of the iPhone Developer Program… Etc. How can I fix this? I've called apple been on the phone for a while

Sync it with itunes

To get on iOS 7, he needs to register his device's UDID with http://www.instantudid.com/. This will get you past the activation error. It's $5 and they take care of it instantly.