For kidney disease low protein tends to be advised by vets so you need to
change what you are feeding since you are giving a high protein diet and
I think that the BARF diet would be at least as bad in this particular
medical situation for a ferret.  This is a situation in which you
strongly need to go by what your vet says because kidney disease is very
unforgiving and sadly often found only when it is advanced in a ferret,
and you need to discuss with your vet what you find that other vets have
done with decent results.  If you think that a non-vet may say something
useful then discuss it with your vet first and get the vet's permission
before trying it.  For something this tricky that is absolutely
essential; it's always important but sometimes it is essential to not
worsen things and this is one of those situations.  We've had tricky
ones and what we've managed to do is to arrange regular talk times if
they weren't bogged down by an emergency, or have call-backs at their
convenience when we've left messages for the vets.  The best discussion
I can think of off-hand on kidney disease is one which is easiest to
find by searching on the name "Champ" in the Complete FHL Archives at
http://fhl.sonic-weasel.org/ ; it contains not only the diet changes
tried but also a range of medical approaches which made a difference for
a longer time than a person usually reads about with ferrets who have
kidney disease, if my memory serves.  Also try other useful words:
kidney, neph (as a partial word to cover a range), etc.  It can be hard
to change diets but I know that others I have read have mentioned making
the same switch before so it is do-able except for the most intractable.
[Posted in FML issue 4150]