If he was doing a backup there will be records stating that the backup failed. It should at the very least indicate what time it started (losing power may stop it writing any more details to the file, it depends on the specifics of the backup.
If he can't show what time (even to within the hour) he can't prove whether it was his own staff or any of yours.
Is it possible that the power cabling could have accidentally been pulled out by moving furniture, tripping over it etc?