Until the phone is unlocked, frankly you have an expensive brick on your hands.
Your "friend" was not being honest with you, so your best course of action here is to return the phone for a refund. Used phone purchases all too often result in a bad ending.
Tell your "friend" that you will consider buying the phone again when it is really unlocked. Assuming that the phone really belonged to your "friend", they will have no trouble contacting the carrier since they were paying them each month for service.
The carrier will not recognize you as the owner of the phone, so they will do nothing to help you even if you can find out who they are. In other words, the carrier will only unlock the phone for the person that they recognize as their customer.
That would not be you. That would be your "friend".