iPhone 4s is locked activation locked after resetting it.

I recently bought a iPhone 4s for my dad off of someone from a Facebook buy,sell,and trade group. I got it home and reset the phone so it was like brand new. I went to re-activate it and it is requiring the email & password that was used to activate the account when it was first used. I contacted the person i got it from and they wouldn't reply to me about them deleting the phone from their iCloud so that i could activate it with no problem. I don't know what to do exactly because I can't contact apple on the phone because of them needing a serial number for the phone, which i cannot retrieve. I need to know how to bypass this step since they will not cooperate with me. Thanks.