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.
Well I havent, but a good friend of mine has. He was backing up his music to his laptop via a usb cord, thought it was through and it wasn't and unplugged the cord. Now he says he has nothing either on the laptop or the multi tracker. He is just sick about it. He had about 200 songs, all played by him, all instruments, tracked over to compose complete songs, about 100 or so were original. What I mean to say is, Hours upon days upon weeks of time and effort. Does anyone know how to help him?