Dogs can go a surprisingly long time without eating, so long as they are receiving fluids. She will not damage her intestines unless this is part of her disease, whatever it is. She is following her instincts in refusing to eat, as digested food gives her liver work to do, which it is not up to at the moment.
I will not pretend that this elderly lady is not very sick, she is, and may not make it. This will not be due to lack of food, but due to the inability of any mammal to survive long without a functioning liver. I hope she does get better, but if she does not, you have my deep sympathy and my praise for taking such prompt action to try and help her.
it is clear cut case of Canine parvovirus (CPV) it's other symptoms which it will show in
three to ten days of getting infected are lethargy, vomiting, fever,
and diarrhea .diarrhea and vomiting will cause dehydration and other
secondary infections and causing this balance in electrolytes and because of bloody diarrhea it will cause anemia and loss of protein.the WBC count drops vigorously and ultimately will lead to shock death . However
it is curable but its success rate depends on how early it diagnose and
usually I.V fluids and colloids , anti nausea injections ,such as
metoclopramide, dolasetron, ondansetron and antibiotic injections such
as cefoxitin, metronidazole, timentin, or enrofloxacin with B- complex
and liver enhancing tonics such as sorbolene , liv.52 etc. Are given
and if the disease is at last stage blood plasma transfusion from a
donor dog which has survived cpv infection .as you have already consult veterinary doctor its good have faith in your vet but also ask him to do test for hb,wbc , esr, lft and blood urea and ask him about possibilities of cpv in your dog.