Q: i bought a used apple iphone 5s and now its locked can anyone help me cause i cant get in touch with person i bought it from

i bought a used iPhone 5s from my friend for 250 bucks and i forgot the passcode that i was given so it locked up the sim card. i didn't have a puk for it since i had forgotten it and so went to at & t and got a new sim card but now the phone is still locked and i cant get in touch with the girl i bought it from so am still not able to use this phone. a t & t said that she needed to go on line and give her info to erase this phone but i cant get through to her on the phone number i have for her so am at a total loss as to what to do. i been trying for over a month now. can anyone help me as i don't want to lose 250 bucks on this situation either. thanks