sick Heidi

My poor kitty is sick. She's 15 years old, I adopted her last year because her owner went into an nursing home. I have another one who is 6, I've had her for a number of years.
I came home yesterday and Heidi wasn't at the door - lately, when I put my key in the lock, I can hear her bell jingle almost right away. So I went looking for her, she was on my bed - usually she is waiting with Deena to be fed. But last night she was not interested in food, not even yummy canned food. She was a bit weak and slumpy.
Same story this morning, so I brought her into the vet's. I was in shock at the vet bill, but what can you do? As cats age, apparently they get either diabetes, kidney or thyroid problems, so they did a test for those. I thought she might have had a cold and have lost her appetite. I nuked some canned cat food and put it on her paw and cheek to see if I could get any of it in her. Since her last vet visit, she's gone from 8 lbs to 7, so I doubt it's just a cold at this point. My friend's cat died of kidney failure, his cat went real real skinny, poor thing. So I figure that she has something along the same lines. Cats can't talk, so I have to act for them, they are vulnerable.
Tonight I'm going to see if I can get some more food into her, or I might try baby food or milk or something.