I was helping someone else to update their OS.
For some reason it wouldn't let them download it using their own Apple ID (in the Apple Store), kept giving a generic error. So I logged in with my Apple ID and it worked, downloaded & installed fine.
Now, whenever they want to get updates using their own Apple ID it says "Update Unavailable with this Apple ID - This update is not available for this Apple ID either because it was bought by a different user or the item was refunded or cancelled."
This is extremely frustrating. How can I allow it to work with their ID or to connect the upgrade to their ID instead of mine?