No, defragmenting harddisks won't do anything. If he's "cleaning" his computer (i.e. deleting "unknown" files) then he might be deleting the activation file. In that case we would need to re-activate (but LimeLM would detect it was the same user and thus wouldn't us another "activation slot").
Also, make sure you're using the latest TurboActivate. Older versions had problems on some buggy hardware.
If he's still getting the problem then we can schedule a time to take a look at it.