Type 1 diabetes is not associated with being overweight and weight loss does not help. Indeed, rapid weight loss is often an early indication of the condition. Treatment with insulin (the only method available) typically results in weight gain, as the body recovers due to the newly available injected insulin. Often these patients start at normal weight range (i.e. BMI < 25) and are often substantially below typical weights prior to development of diabetes.

Type 2 diabetes has strong genetic factors and substantial evidence shows these, combined with weight and some other factors of diet, lead to an expression as diabetes. In these cases, some people have had significant improvements in their conditions due to calorie restriction (leading to weight loss but often it also means they've switched to a healthier diet in the process and that also helps). The mechanisms here are believed to be due to improved sensitivity to insulin which is still being naturally produced.

Various ratios of diabetes are quoted, but generally 1 in 20 of the white european population has diabetes and 1 in 20 of those has type 1 (and the other 19 type 2). Figures for Indian/Pakistani origin people are dramatically different if they move to Europe or North America and research suggests this is due to very different foods consumed [the 'western' versions being significantly less healthy than those from origin countries]. Much higher numbers of type 2 diabetes are found in these populations (reinforcing the factors as both genetics and types of food are involved). Other population groups differ too.

Treatments for diabetes typically create weight control issues as increased insulin or sensitivity to insulin promotes body fat storage.

It's untrue (and frankly verging on offensive) to simply blame diabetics for their condition because they are obese or overweight. Addressing and treating the condition is significantly more complex and weight control may be one of the few elements the patient has any input to, while many other factors are entirely outwith their control and they bear no culpability for.

I guess I agree with you, then.

Some studies indicate that weight loss is great for fighting diabetes : http://link.springer.com/article/10.1007%2Fs13340-013-0107-1

Conclusion

Six-month obesity treatment of obese type 2 diabetic patients led to significant weight loss and the amelioration of glycemic factors. For obese type 2 diabetes, more than 3 % weight reduction is needed to improve glycemic control, and more than 15 % weight reduction and, in addition, short duration of obesity and diabetes (3.5 ± 2.5 years), are required to normalize glucose tolerance.

Some one who is "pre-diabetic" is well advised to eat right, work out and lose weight. That is their responsibility.

Check out ProgrammerGirl's censored post below for another study.

Don't be offended by advocating for personal responsibility in fighting this disease.

PS - Study recommends a ketogenic diet which I am very much in favor for. It has done wonders for me (ymmv). But I am still on the fence about being "overweight" being a direct cause of diabetes and not an effect.

The parent did not bring causation into the discussion, you did.

You're right to doubt it. Metabolic syndrome is the common theme http://www.nhlbi.nih.gov/health/health-topics/topics/ms and it likely leads to both obesity and diabetes, among many other ailments.

Of course the type 1 disease which is ~10% of diabetes is different.