I wouldn't think so. If the younger cat was still really a kitten, under a
year, I might be worried about her getting enough nutrition, but she's
adult, and should just eat more of the diet food. But if your friend is
worried, why not feed the cats separately, or put the thin cat's food in a
box with a small entrance? It won't matter too much if she eats some diet
food in addition to her regular food; the problem appears to be keeping
the fluffy cat out of the higher-calorie food.

Isn't it always the way that cats never eat the stuff you especially want
them to eat - the diet food, the vet-recommended special diets, the
expensive new food that's supposed to be so much better for their health
than what they prefer....

I don't know about "harm", but I know it's definitely not what's best for
her.  Kittens need concentrated nutrition because their little tummy's just
can't hold enough "regular" food to provide for their growing bones and
muscles.  Personally I would rather see my overweight cat eat the kitten
food than the kitten eating the "lite" food.

Hugs,

CatNipped

Shouldn't harm the baby. If the little one starts to look skinny then
your friend might want to change something, but it didn't hurt my Samuel
or Tabitha to switch to adult "less active" food.

Dan

I wouldn't worry a lot about it, since the little cat is old enough.
What I would suggest is avoiding dry food altogether and feeding both
cats a good quality wet food.

Dry diet food generally has more fiber and less carbohydrates; and the
kind I feed, Nutro Natural lite, has more protein; which a growing cat
needs. Many cats have their growth before they are two. But some
breeds like the Maine Coon are slwoer maturing. Cats really don't need
all those carbs since they are obligate carnivores. Most dry foods are
loaded with carbs. Lite food is the best combo for the price. Of
course there are much more expensive foods that are good. But Nutro
serves my cats well. One is 13 and the other is 2.

My diabetic cat (14) eats only canned food with no carbs. Look at the
ingredients. You should see only meats, fish, and vitamins. No food
starch or gravy. If you are worried about the young cat, you can
supplement his diet dry food with a dollop of canned kitten food. But
of course only when the overweight cat is not around!

--
CATherine