In order to remove a device from Find My Device, the device must first be erased.
I think the owner was trying to make a time machine backup in order to do that but couldn't figure out how.
I don't think the new owner is very tech savvy, I told them that this was a serious breach of privacy and security but I'm not sure they understood or cared.
I told him he should take this up with Apple and or Newegg and they would almost certainly replace the computer because this was so crazy, but he seems reluctant to for some reason.
I don't know if I would file this in the 'no biggie' folder
If this happened to someone and the previous owner was a bad guy, they could really screw things up. Data ransom, GPS tracking, etc.