Here is the usual sequence for relinquishing ownership of an Apple iPhone:
What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support
Use the no-longer-have sequence.
Given it’s trivial to take over an Apple Account from a trusted device, you could have lost (or did lose?) control of the Apple Account. Which is a larger problem.
If the iPhone is tied to their account, you’ll need to present proof of ownership to Apple, and request Activation Lock be removed. This if the person holding Activation Lock don’t follow the above-linked sequence to relinquish their iPhone ownership to you.