Usually backups are done on a file by file basis. So if you unplug during the transfer of one such file, only that file can be damaged.

And usually also, the source of the backup is only read, not updated. So whatever you do, it can't be damaged and it can't be deleted.

So - if the story is true - he must have used a very strange backup program. As you don't even tell what program it was and how he used it, it's impossible to say anything more about it.

Finally, let me remark that making zero backups during "weeks of time and effort" isn't a very good practice. If you make one every day, you lose at most one day of work.