I gave someone (he is not very experienced working with audio) 24 bit aif
files on a CDR. I'm using a PC, Samplitude 7.11. He copied the files to
his
Mac and says they a
A) "Not even recognised" by ProTools or Logic (he didn't say what
versions),
and
B) Show up as 16 bit Quicktime movie files.

I told him to try renaming them "aiff", because I have seen this same
problem fixed that way before, but he insists it doesn't work in this
case.
He is in London and I'm in S. France so I can't just pop over there and
try
a few things myself. Any tips?

Sounds like he's still using OS 9, which still isn't a bad idea for audio.
Tell him to look for a program on Versiontracker.com that allows you to
change creator and file type settings. Last one I used was called Creator
Changer for OS 9 (worked in classic). The creator shouldn't matter, but the
file type should be AIFF. If that doesn't work he should try opening it
with a sound utility like SoundApp (also for OS 9), and do a null
conversion, meaning no file attributes are changed, but a new file is
generated from scratch and should be properly formatted. Peak can also work
for that.