The obvious advice (which I am sure she is adhering to) is to watch her diet strictly, check her blood sugar often (as in daily), take her oral medications and/or insulin to regulate her blood-glucose levels, exercise and be careful with minor infections (cuts, scrapes, open wounds that can become infected very easily and possibly lead to amputations and other complications).
Since I am sure she already does all of that, because judging off of your personality here, I would doubt that she were anything but level headed, I will instead advise something completely different and unconventional that might not interest you, but could possibly work.
The problem with Diabetes involves the underutilization of glucose via too small of an amount of insulin secreted by the pancreas or an overall resistance to insulin by your body's cells that can lead to a hyperglycemic state.
That said, why not place her on a ketogenic diet with very little caloric restriction? (Now hear me out)
If the problem is the utilization of glucose due to underproduction or resistance of insulin, why not just cut out the middle man all together and have her use fats for energy instead of glucose?
The ketogenic diet forces the body to use fat for energy and by cutting out carbs, her blood glucose levels will decrease (taking care of the diabetes issues, for the most part) and while she can lose weight due to the process, you can simply not restrict her caloric intake and simply watch the type of fats that she consumes in order to keep cholesterol in check (High HDL and low LDL and TriGly).
I'm not diabetic specialist or anything of the sort, but why not propose this to your mother's doctor and see what he thinks. At the very least, you can present it to him with the facts and if he knocks it down, let us know why.
That said, there are many studies backing the use of a ketogenic diet for the treatment of diabetes. Weight gain can be a little more difficult, but that can be handled as well.
This is just my opinion, feel free to point out any and all flaws..
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