Do you know how to unlock a iPhone that is connected to its owners phone

You are not able to unlock it without the original owners Apple ID/Password. The device will never be usable to you. This is a theft deterrent and if there was a way around this it would defeat the whole purpose of the owner of the device being able to lock it to prevent access from others. You should be aware that the original owner is still able to track the device to your location if they want to.

Return that iPhone to the owner. They can enter the passcode to open the phone. No one else can.

If you bought the phone, return it for a refund unless the owner can unlock the phone for you. The phone is worthless otherwise.

To unlock that iPhone you need its passcode and its Apple ID, if it's been setup a two-factor authentication you also need a code