Given the family history of your friend and the fact that she is morbidly obese, she is probably at high risk of Type 2 diabetes. Eating lots of high GI carbohydrates will not help. High GI carbohydrates are rapidly converted to glucose. This resulting glucose spike leads to a corresponding spike of insulin - this can lead to what is called insulin resistance which can progress to Type 2 diabetes.

Mutations in many genes are risk factors for Type 2 diabetes. The effect of a mutation in just one of them is usually small but this is cumulative - the more relevant genes are mutated the bigger the risk. This is because the more different abnormal proteins (coded for by the different mutant genes) somebody makes the more likely it is somebody will develop Type 2 diabetes.

You and your friend might find this helpful Genetics of Type 2 Diabetes.