Sign in to their iCloud account atw

Sign in to their iCloud account atwww.icloud.com/findClick All Devices to open a list of devices linked to their account, then select the device to be removed.Click the Erase button to erase all content and settings from the device. When prompted, don't enter a phone number or message. Click Next until the device is erased.When the erase is complete, click "Remove from Account" to remove the device from the account.

After the device is erased and removed from the account, you can proceed with the device setup process.