Actually, I think what has happened is that he has been importing the same course for many years and he just re-uses the activities. The trouble then is that somehow with each use the gradebook gets more and more items in it.
I tested importing his course but unchecking all the activities and suddenly instead of dozens of orphaned gradebook items you get a clean and empty gradebook. I also tested importing all the activites and then deleting them after the import, but the gradebook is still cluttered with dozens of items. At least those items can all be deleted and there aren't any of those weird un-deletable items.
This seems like a problem if you ask me, but moodle seems to like things to be horridly messy.