How to Completely Remove Apple ID without Password?

I'm using an iPhone 5s Global (CDMA) equipped with iOS 11.4. This iPhone belongs to my Co-worker and I bought it for Php 1,500.00 (around USD 55.00) which it has an existing Apple ID associated on the device owned by him. The problem is he forgot the Apple ID Password and can't recover the account because his current Trusted Phone was lost or maybe he forgot where'd he put his sim. His current Apple ID was associated by an email address, but it is too a problem, he forgot the password.

I tried a lot of things, Unlock Apple ID without a password using a third-party application, like (Tenorshare 4uKey) which is useless because I'm using an iOS 11.4. I don't want to try to enter in recovery mode using iTunes because there might be a problem when it ask for "Activation Lock Password". I can't turn off "Find My iPhone", it always asks me to input the associated Apple ID and Password in it. Also I downloaded the Firmware Package of the phone but I tried to avoid that step for now, I'm not certain for the upcoming uncertainties to happen.

I'm stuck and can't find the best solution for it.

It is impossible…
It's a main security and anti-theft feature and would be pointless if it could be removed.

The account owner CAN recover their account through the AppleID web site, if they can be bothered to go through the required procedures.

The phone ID data is linked to the owners account on the Apple servers. When you reset one etc. It must connect to the Apple servers to verify the person that has that phone has access to the account it is linked to.

Nothing you do to the phone itself can change that.

(I believe there have been some tricks/scams in the past that make the phone itself think it was authorised, for that specific activation - but the next time it checks the account on the real servers, it would be de-authorised again).

Simple. You can't.

Activation Lock Can't be bypassed.

What you paid for is utterly useless. Ask the seller for a full refund, but the seller can refuse as it was your job to thoroughly check that the phone was usable BEFORE you paid for it.