parakeet on death's doorstep

[Amadahy]

Today I woke up to find my 8-year-old pet parakeet fluffed up on the bottom of her cage, hiding under the water/food cups and completely unresponsive. She let me touch and pick her up without any fuss, which she hasn't let me do in years, and she has discolored diarrhea. She also weighs almost nothing.
I've noticed that she's been fluffy since early September and I knew she was cold, but there wasn't anything I could do about it.

I now have her in a dark, heated, wrapped up box with food and water in the quietest part of the house I know of. My mom likely will not take her to the vet because the last one we took cost over a hundred dollars in antibiotics and died anyway. She's bitter about that...

Is there anything I can do to help my bird?
I know I probably can't prevent her from dying; I just don't want her to have to suffer like this.

[Deleted]

@ama
I've never had much experience with birds, but definitely try talking to your mother. If not to try to save it, then to put it down. If she still doesn't go for it, then at least make sure that she's comfortable. I definitely think the set up that you have going right now is the best thing you can do for her right now. I'm sorry your bird has fallen ill, I hope things work out for you, dear.

[Re Harakhti]

You're not taking it to the vet?.....
The day I was due to take mine he actually died so I never got the chance to help him.

Research online and try changing the diet. Keeping it calm and warm is the best you can do, though it might feel better at home in its cage, just make everything more accessible and blanket the bottom of the cage, and the back and sides so it's softer/more comforting.
Less handling is definitely better for a bird who generally isn't used to it, because stress will make things worse.