Those old cables were going to short out at some point.
Just replace the cable, these are pretty easy to take apart.
Woah... faulty driver, that's terrible. Pretty sure it's impossible to repair these.
These are too rare to find spare drivers lying around.
You might have some luck posting a 'wanted' advert in the 'For Sale' forum for a spare driver, but I wouldn't have too much hope of ever finding one.
It's not quite impossible depending on the situation.
What he's got is likely a failed connection to the voice coil. The solder pads on this headphone are poorly designed in that the voice coil contacts them on the underside of the PCB, which is glued into place and can "shift" around due to the poor aging behavior of the adhesive. Simply moving the headphone around a lot (such as would happen from normal use or shipping) can cause the pads to loose contact. Cable tugs and jostles aren't particularly safe either due to how little space there is in the earcup.
This issue occurred on both my Z5 and Z6, but I was able to repair them with some extremely tricky soldering (though I only managed to fix the Z6 driver after parting out the rest of the headphone, bah).
The simple solution is to accept that these are a home headphone and never take them outside despite their small size, to keep them in a safe spot (headphone stand, etc) and to be as gentle as possible with them. Metal housing or not, they are almost 40 years old.
From my old observations of my working D7 I would never have never guessed it was so simple - especially since one would never under normal sane circumstances pry open a working D7 driver to see how it's put together
Sounds like an easy job if you've got patience and a good magnifying glass.