Many thanks for the reply. I managed to install KIES but the PC still generates this error message, but with KIES installed it gets into a loop and keeps presenting the message.
I'm able to connect my SGS4 to the PC and it appears immediately as a browsable device in Windows Explorer. (He's running Vista 32bit.)
I think that something has been changed in the process of unlocking the phone from Orange, probably as a result of him accepting the denial log service, meaning he'd installed Samsung Knox. The bloke who unlocked his phone has had to do a fairly low-level change, possibly even rooting the phone, to get it unlocked.
Is there a simple way to see if a phone is rooted?