You MIGHT be able to activate it via iTunes or even the carrier but if anything is wrong with it, even if that person told you it was under warranty it is not. So you will be hosed.
My guess is either the person got it replaced and then palmed the replacement when someone wasn't looking, or pulled some kind of serial number scam on the tech. That crap is part of why Apple changed rules demanding techs open all phones that won't turn on to find the serial inside and require a validation of that serial with the logic board serial (they are paired in system to know what should be together) and removed the imei and serial from sim card trays in newer models
I wouldn't take the risk, for all you know it's on iOS 7 and activation lock is in place and the guy will disappear leaving you with a brick that no one, not even Apple, can sort out