I need to return someone's lost iPhone

It's great you want to return someone's lost phone. It really is. Unfortunately, this is a user to user only forum and we can't help. And Apple doesn't get involved in lost or stolen goods, so they can't help.

What you should do with the phone is turn it over the authorities where you found it. Hopefully, someone filed a missing phone report with them.

Again, thank you for wanting to do the right thing.

Return the iPhone to the lost and found of the establishment you found it or turn into local law enforcement.

It is only the owner of the phone that can choose to reveal any personal information. They can include contact details when found and it will appear on the Lock Screen and they can also continue to track the device to know exactly where it is.