My first question is whether the diabetes is Type I or II. Type II is generally an insulin resistance issue which can be helped with exercise.
If it is Type II and they are overweight, then a regimen focused on weight loss is helpful. This is due to the link between obesity and insulin resistance. But resistance type exercise is also useful because it enhances the muscles use of glucose.
Also exercise will enhance the effect of insulin. So they may need to take less, especially for type I. Most diabetics already now this, though.