Barn kitty w/ poss kidney failure

A couple weeks ago, I asked about a diet for the kitty at the barn where we board. She has gotten thinner. She is at least 15 yrs old and has been in very good health. Hyperthyroid or kidney failure were mentioned.

So I bought some kitten food for her as was suggested. I am just wondering if the higher protein in it will work against her if it is kidney failure. The protein number is 40. I bought some 9 Lives Long Life Formula for maturing cats. The crude protein number is 31% which I am more comfortable with.

I certainly don't want to do her any harm.

What do you all think?

Thanks

We can't guess what will or will not be good for her unless we Know if it's kidney failure.
Have you taken her to the vet to be looked at?

If you suspect kidney issues, first thing to do is to switch her to all/mostly wet food (which wouldn't hurt even if it's not her kidneys). But without knowing what she has, it's not possible to make specific suggestions about what's good and what's not.

My Fafeena (17 y/o farm kitty) is in renal failure. She gets 1/2 of a can (5 oz size) K/D canned food 2x/day. Dry food is harder w/ the other outside kitties, as they are free fed basic dry food.