UPDATE:
The friend was overwhelmed by the various ways of trying to find the original owner, so he dropped it off at my place this morning and said I could try whatever I wanted to.
I first tried tracking him through the IMEI number, but that proved unsuccessful. I only found out that he had gotten it blacklisted. After various trials and errors I happened upon the brilliantly obvious idea of googling the number the phone said to contact. It brought up a facebook name. When I had plugged the phone into iTunes, it showed parts of the email address of the person's Apple ID. The email on the Facebook page matched those parts, and from there it was relatively easy to contact him. We met at a Target and justice reigns.