It's very hard to tell with so little info what is going on, but dilated pupils could be due to fear or stress. The bottom line is that a 15 year old cat who isn't eating well needs a vet asap. Can you take her in?
My kitty wasn't eating or drinking either. How is her breathing? Mine ended up having congestive heart failure. Might be time for a trip to the vet. I wish you and your kitty well.
I took her in and was told gently. Nothing they can figure out without doing extensive testing which at her age not suggested. We of course did all linda of blood tests. And I am picking up a steroid to see of that helps. Praying I have the courage and wisdom to right by her.
Can your vet give her an appetite stimulant to encourage her to eat a bit more - you can see what she/he thinks of this idea (whether it would be reasonable). It would not pinpoint the problem, but might stop her getting fatty liver.