Originally Posted by
Heisenberg Something makes me think that that backup is corrupted.
I'd agree, but it's already two backups. When he backed up to the cloud, it wouldn't copy from his iTunes. It'd create a new backup from the phone.
Do you have any other devices available?
Have you tried restoring that backup on the 6? Does that work?
If that works, have you tried making a backup of the 6+ and see if that can be restored on either device?
I remember when the 5 came out, there was a special iOS version specific to the 5. I haven't paid close attention to iOS upgrades lately. Is the 6+ on a latest version that is versioned slightly higher than the 6? As an example, 8.1.1 instead of 8.1.0.