That's simplistic and based on assumption ArgyMargy
First of all, the OP does not say MIL is overweight. She has heart disease with high cholesterol. Some types of heart disease cause arteries to collect cholesterol as a response to damage rather than dietary excess. She may be overweight as well but it's not a given. Either way, heart disease makes you more prone to diabetes.
I am not overweight, far from it. I am 7 stone 12, with very little body fat. I cannot afford to lose weight. Nor do I have much sugar in my diet to start with. In my case I have been told it's likely to be autoimmune as I already have illness that affects my immune system.
The debate with regard to diet is which type of diet is best at getting blood glucose levels down. The old NHS advice is, as I said fairly heavy in carbs, which as I'm sure you know produce sugar. Newer developments indicate that restricting carbs and increasing protein may be more successful at lowering blood sugar levels. On the other hand, too much protein can cause kidney problems and osteoporosis.
For me, I can figure it all out myself from books, Google and medical advice. But from OP's account of MIL, she's not going to do that. By the sound of it dietary advice has to come from a medical professional otherwise she won't take it seriously.