How do I remove previous owner's Apple Account from secondhand iPhone?

Activation lock is a security protocol that can't be bypassed. Only the person it is locked to can unlock it.

If you bought this from someone and it came locked, try to get your money back, you will never be able to use the phone.

Something isn't adding up here, why would there be a police report if it was supposedly bought on the up and up. In fact, why is there a police report at all?

Usually when the previous owner can't be contacted or the previous owner can't remember their passcode, these are good indicators that the phone is stolen. I think your son's very good friend just ripped you off.

If your phone is Activation Locked - How to remove Activation Lock -

How to remove Activation Lock - Apple Support

The All Too Common SAD Reality of Buying a used phone -

The All Too Common SAD Reality of Buying … - Apple Community

Hello~ Even a police report will likely not satisfy Apple’s strict security requirements. This iPhone is probably never going to be unlocked as the Activation Lock is doing it’s intended job of protecting the previous owner’s information. Unless you can contact the owner who’s Apple Account to which this iPhone is locked to …you may as well recycle it responsibly. Click on below and review…

Activation Lock for iPhone and iPad - Apple Support

How to remove Activation Lock - Apple Support

Also…

The All Too Common SAD Reality of Buying … - Apple Community

~Katana-San~

No Police Report will help with anything. The phone will remain a brick unless the original owner is able to remove the device from their account. There is nothing Apple can do, and they do not get involved in any third party sales. You will find that when there is no contact with the original owner and there is no way to get a refund, you just need to chalk it up as a valuable lesson learned and never do that again.