Sick and thin pup won't eat

my mom just recently (about 2 weeks ago) adopted a dog (believed to be miniature pincher) who developed kennel cough. She was really thin to begin with since she's a rescue but she is still so bony. Her cough is better, she only just scratches at her nose and sneezes a bit. She is on antibiotics but she's soooo lethargic and it's so hard to get her to eat anything. My mom left her with me to take care for for the next two weeks while she is on vacation so I want to do all I can to nurse her back to health. Is there any more that I can do to get her to eat and gain weight and energy? The only thing I can get her to successfully eat is two or three finger sized pieces of liver. She will drink water but only with encouragement. She will only walk around momentarily before she finds a place to lay down and sleep, sometimes it is difficult to get her to wake from sleep or open her eyes.

Again she's been to the vet 3 times, is on antibiotics, probiotics, and vitamins. She just doesn't seem to be improving very quickly and I'm its sure because she was underweight before she got sick. She needs energy fast, how do I get her to eat? Is it ok to keep feeding her as much liver as she'll eat?

Do not just feed her liver. Liver contains vitamin A which becomes toxic if it builds up past certain levels. Vitamin A is a fat soluble vitamin and therefore is not cleared out of the body via the kidneys. Liver should only be fed in limited amounts to make sure that the dog's body has time to clear the vitamin levels.

I would try enticing the puppy to eat a good quality canned food.

It could be that the antibiotics are also upsetting the dogs tummy causing it to not want to eat.

If you need to take the puppy to a different vet and get a second opinion!

Also how old is this pup?

I would second the 2nd opinion suggestion! A dog who is over kennel cough should not be that lethargic. I'd be worried about another underlying condition and/or pneumonia.

Dogs who are congested can't smell as well which means food isn't as enticing. Sometimes adding warm water or zapping the food in the microwave to lukewarm will help emphasize the smells.