The first thing your friend needs to know from the vet is: was the 'bump'
benign or not, what the chances are of a recurrence, and how strong the
kitten's heart is.  If it's general health is good, OR if compromised, your
friend is prepared to provide lasting care, then I see no reason why a
retarded kitten can't be happy and have a good quality of life.  All this
depends on the degree of retardation, but stimulating play and exercise will
make all the difference.  To me, the expression 'somewhat retarded' implies
that, perhaps the condition is not too bad.  This is, after all, a very
young animal who has time to improve.  Advise your friend to play with the
cat to test its responses.  If these give hope AND if the cat is able to
control its own toileting, then your friend should be able to cope with and
care for her kitten.  (She must be prepared to handle the cat's grooming, if
necessary).  In the worst case, be a true friend and offer to go with
her/him to the vet when the kitten is put to sleep.  This is the most
difficult, but most loving, decision we have to make for our furry friends.
I've had to do it; it was heartbreaking, but made easier by being surrounded
by friends.

I should add that a friend of mine had an autistic cat for years.  It
outlived her and, sadly, had to be put down (at a good old age) because she
had no carer.  There really is hope.

Good luck.
Spider

Did the vet ever say it was an allergic reaction that caused the arrest. As
for being a little off, it will take six months to a year to really know if
it's permanent. Kittens can have a turn around in no time from things like
this. Also they may have to up the kittens *Taurine intake for awhile, just
to help with the heart and neurological development.

Shadow Walker

Taurine:
http://www.serve.com/BatonRouge/taurine_chmr.htm
http://www.petcaretips.net/cat_taurine.html