There are various books - though I have mainly used diabetes forums. But I started with something extremely simple and completely free.l
It is the Sugar equivalents of common foods (endorsed by NICE). Most people know that Type 2 Diabetics should cut down on Sugar, but these show that carbohydrates contain huge equivalent spoonsfull of sugar since they almost immediately break down into sugar.
I think this is simple enough for an 80yr old to understand.
https://phcuk.org/sugar/
You can print them out for thee patient.
Rather than read a book like 'The Diabetes Code' by Dr Jason Fung, you could just give them this simplification:
The human body can hold very little sugar in the bloodstream.
Too much sugar in the bloodstream is very bad.
Insulin (the fat regulating hormone) is produced in order to remove excess sugar from the bloodstream. It shifts it into muscles for use as energy, but also into fat cells for storage of energy for future use.
Because Insulin is primarily removing sugar from the bloodstream, it also prevents the body from 'burning' its stored fat as energy - so that the body uses up the sugar instead.
(All the above is completely accepted by mainstream medicine).
The next part seems obvious (to me) and has lots of evidence to back it up, but many doctors don't yet accept it:
Because high Carbohydrates in meals cause high Insulin, and Insulin increases stored body fat, the following 2 things are also true:
1. Type 2 diabetics should eat much less carbohydrates.
2. It is almost impossible to lose weight if you eat lots of 'so -called healthy carbohydrates' - such as whole grains.
So what should a Type 2 diabetic eat? - Replace the calories from Carbohydrates removed from the diet with 'traditional fats' - that means Eggs, Butter, Cheese, Full Fat Yogurt, Double Cream and fatty cuts of meat and fish. - Just what that 80yr old would have eaten, if parents could afford them, as a child!
Just to note that Dr David Unwin (a GP in Southport) has between 40% and 50% of his Diabetes Type 2 patients in remission = on no medication and HbA1C at least down into 'pre-diabetic' range.