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Since iOS 7, you cannot restore iPhone which had "Find My iPhone" enabled and linked to the user's iCloud account. Unless of course you have the iCloud account password. Another option would be to go to the Apple Store and explain the situation (i.e. company phone not belonging to employee who left etc.). They might be able to do something about it.
He wants to wipe it clean, not just restore it. I think if he follows the recovery mode advice he should be good to go.