Activation lock, asking for proof of ownership even after submitting the original bill for the newly purchased phone

My friend had purchased a new iPhone last year and the seller has created and gave him a new apple id and written and gave him all the information about the account on a piece of paper except the apple id.

After using the phone almost for a year, he did some updates on the phone, and later on, it was asking for the apple id which he doesn't remember. He tried logging in by guessing the email id and end up getting an activation lock. We contacted apple support and they advised us to send a copy of the original bill. We uploaded a copy of the original bill on the link that has been sent by the apple support team, they informed us that the bill should be with serial number and IMEI number and to check with the seller to provide the document. We did that as advised and got a new bill from the seller with the serial number and IMEI number, uploaded it to the provided link. Even after that from apple support, they are asking for more documents as proof of ownership. We visited the seller again and they informed us that there are no other documents they can provide other than the original bill and that is only the proof of purchase for the phone. Again we tried contacting apple support to have a solution for this and they were advising us with the same message that they need more documents to verify this.

Do let me know if anyone can help me resolve this issue as the phone is becoming of no use for the original owner.