My 15 year old Irish Setter mix has not eaten since last Wednesday. Had diarrhea, very lethargic, wobbly back end. Husband took her to vet yesterday and got a diagnosis of flu or giardia. Vet gave is pills for diarrhea which she doesn't have anymore because of nothing in her system. Drinks small amounts of water but struggles to get up and move. I'm freaking out because she's my "baby," What could it be -- I can't get her to eat or anything!

Your dog needs to go back to the vet. Did your husband decline treatment in the sense of IV fluids? It really needs to be rehydrated..

No, he did not decline anything. We have 4 dogs and they are perfectly kept and vet checked, vaccinated always. Vet indicated Caya was still hydrated yesterday so nothing was mentioned about IV fluids. They asked to bring in a stool sample; however, she hasn't pooped so we have nothing to sample. Don't know what to do!? I just force fed her about 1/4 c of bacon with brown gravy.

First things first, don't feed her bacon and brown gravy, her pancreas is probably objecting as we speak, a dogs digestive system doesn't handle fatty foods like we humans.
But you will likely get your fecal sample once that bacon shoots through her.
If you are going to try human foods stick with boiled chicken or hamburger and rice, drain the fat, no skin, no bones, no seasoning.
Was there any testing done by the vet. If not why? Blood work, X-rays?
Typically a fecal sample is easily obtained (there are ways) unless the dog is completely empty. So was there a fecal sample or was giardia a guess?
At 15 she is well up there in years, but I have seen a fair share of pretty healthy older dogs, with problems that can be dealt with.
But that's the kicker you have to figure out what those problems are, honestly if your vet offered you no more than what I am seeing in this post, then I would seek a second opinion.
If she/he did and no answers were found, I apologize.
If she is drinking that is great, but you can help to keep her hydrated with gatorade or pedialyte.
Unless I am way off base here, second opinion.

Thank you. I agree. Just need validation, I guess. On our way...
Will let you know outcome.

One more thing. No fecal test. Asked us to bring a sample in (unable to get one) No blood work. Guessed at flu or giradia. She is old but has been very healthy until now. It's killing me.

Good deal, I hope all goes well for you.
Best of luck to you and her.

Understandable that you are upset, you don't care for an animal for 15 years and not get upset when something happens, they are our family also.
I do so wish a favorable outcome for you guys.