I believe lifestyle choices as well as other modifiable variables can affect the health of DMT2 patients.
A disease which increases the risk of having DMT2 is something called metabolic syndrome (MetS for short). There are 5 criteria for the clinical diagnosis of MetS such as central obesity, triglycerides level, reduced HDL-C, elevated blood pressure and elevated fasting blood glucose. Only 3 out of 5 criteria is required to diagnose someone with MeTs, and once a patient fulfills 3 criteria, it's very likely they have DMT2 too (but of course they'll still need lab work done such as HbA1c levels).
If a patient has diabetes and fulfill these criteria and even surpass these criteria, it's very likely their health and quality of life will reduce significantly. For instance, let's take a look at central obesity. The threshold is >80 cm for females and >90 cm for males. Let's say there's a patient who has a waist circumference of 150 cm (I use 150 because I know of someone who's 110 but moves a lot). All that fat will make movement difficult, and if the patient does not make an effort to overcome that difficulty in movement, fat won't be burned and the patient's weight can only rise from there. I would also like to mention the amount of fat surrounding the organs and inside the circulatory system, which can cause serious health issues.
Other criteria of MetS such as triglyceride level can cause the blood vessels to be thick (atherosclerosis) and can cause high blood pressure (another one of MetS' criteria) and even stroke. Elevated fasting blood glucose has a name and it's hyperglycemia. People with hyperglycemia are usually also diabetic, and if blood sugar levels keep increasing in someone with diabetes, the health of many different organs will get worse. Wounds will heal much slower, gangrenes may happen, the eyes will become blurry, and so on. Those examples were observed directly by me since I had a housemaid with DMT2, but you can also easily find sources regarding the effects of hyperglycemia.
So for DMT2 patients, you would think that a low-fat diet is great, but literature reviews have found out that the low-calorie Mediterranean diet is the paradigm for MetS prevention and should be given for DMT2 patients. For Asians however, low-calorie diet without Mediterranean aspects are just as good.
For further reading and statistics I recommend reading this article, "Dietary Strategies for Metabolic Syndrome: A Comprehensive Review" https://www.mdpi.com/2072-6643/12/10/2983
Lifestyle changes are excellent for controlling the blood sugar levels, but you also need to pay attention to dyslipidemia and hypertension. The most effe3ctive lifestyle change is to get patients to stop smoking cigarettes.