Hard to tell. Only the carrier that the phone is locked to can authorize an unlock. Purchasing the phone from a reseller does not guarantee that phone is officially unlocked. Your friend would have to find that out. If the phone is hacked to unlock, as soon as you try to restore the phone for yourself, it will get locked back to the carrier it is locked to. Before he sends you the phone, he better make sure that Activation Lock has been removed, and that it is officially unlocked. This explains Activation lock. http://support.apple.com/kb/PH13695
Cellular phones operate like a radio, and use cellular frequency bands to operate.