Again, many thanks, hellsbrink.
Downloaded the isobuster and installed it; looked at the problem CD and it 'saw' the folders and the two recording sessions but couldn't extract the files.
The pop-up box suggested purchasing a licence but that would seem to negate the point of a 'free' trial. I'll see how desperate my mate is for recovering his data. The users opinions appear to be universally positive so it might be worth it.