As you may know we are not medical profeesionals on this forum and we can only offer advice based on our own experiences or aid you by doing some research. I am sorry to hear about your mother's condition. I know it must be difficult for her and for you as well to watch her go through this. I looked up some authoritative sites about autonomic neuropathy in diabetics and I basically found information explaining what is called "diabetic diarrhea":
"Diabetic diarrhea - is autonomic neuropathy that results in an erratic functioning of the small intestine. This can cause unformed stools to be passed. If the nerves which communicate with the sphincter muscles (which control passing a bowel movement) are not working properly, stool can pass without warning, and/or without being able to control when it comes out, resulting in fecal incontinence."
source: https://store.joslin.org/managing_your_diabetes_600.asp
And as for the treatment, some sites mentioned medications in general, including an antibiotic tetracycline, etc. However, you say that your mom has tried all kinds of medications and they don't work. Sadly I can't find any other forms of treatment for this condition.
Is your mother's diabetes under control? I don't know how much of the damage done by autonomic neuropathy can be reversed but perhaps good blood glucose control can minimize some of the symptoms. You may also try and get a second opinion. Not to say your mother's doctor might be wrong but doctors are people too and they can't know all there is to know about everything, so that's just another avenue to explore. Please tell your mom to hang in there and not lose hope. I'm sure something will be able to help her. Good luck and take care.
Autonomic neuropathy is a common complication of T1 diabetes; I have had it for 30 years and have had issues with gastroparesis causing slow stomach emptying, slow digestion and bowel problems. It is a complex problem of a complex system, and it must be treated by identifying what part, or parts of the digestive system that are most involved. If she has not done so already, your mother should see a gastroenterologist experienced enough with this problem to target the root cause of her diarrhea and find the correct solution.
In my own case, I suffered for years with over full stomach, vomiting and alternating constipation ad diarrhea. I have none of that today. It was corrected by diet and medication, though all through my own efforts of trial and error. A diet without slow digesting foods (red meat, high fiber vegetables, etc.) that is standard for gastroparesis helps the most to ease the digestive process. I also take a smooth muscle relaxant that helps the constipation (Cialis, in my case, for its dual function) and Alpha Lipoic acid, which has aided my peripheral neuropathy, and may aid the autonomic neuropathy as well.
If you mother has primarily diarrhea, then other treatments may be appropriate for her, but there should be a method to stop this problem once the root cause is known. Foods with laxative effects (like Sorbitol), overgrowth of intestinal bacteria, etc. should be ruled out. Autonomic neuropathy is a broad complication and affects everyone to a different extent and in different ways. It is not a diagnosis for the resulting problem, any more than "poor health" is one. It only indicates where to look for the specific issue. She needs to see someone who is willing to find her specific problem and solution.
See the following links:
http://www.medscape.com/viewarticle/473205_5
http://www.uspharmacist.com/oldformat.asp?url=newlook/files/Feat/ACFA4F4.html&pub_id=8&article_id=793
http://www.mayoclinic.org/news2003-rst/1733.html
http://www.diabetessymptom.net/news/news_item.cfm?NewsID=263
http://www.gicare.com/pated/edtgs08.htm
Good luck to your mother. She should be able to beat this, and any other complications of her diabetes.
Larry