Apple Watch Problem

Hi I recently sold a buyer my used Apple Watch. I completely reset it and it was no longer connected to my phone or hooked up to Find My. However the buyer says that the Apple Watch cannot pair to her iPhone. They say that the watch says this “If you were given this device or purchased it second hand, you need to contact the owner to have them remove this device from their account.” I need help on what to do about this because we both don’t know how to fix this. I have been to iCloud.com and looked at Find My but the device isn’t there, so I can’t turn off the activation lock if that was the problem. I would definitely appreciate the help.