Why are you posting and not your father? Can you explain "precautions"? Do you mean prevent his diabetes from progressing?
If yes we need to know what he is doing to control and manage his diabetes - foods he eats, exercise program, blood sugar levels and when he tests?
"the good diet plan for them...."
Changing lifestyle habits can go a long way in controlling type 2 diabetes. India leads the world in diabetes. This is mainly due to poor diet/nutrition.
Things he must do: Restrict & avoid refined sugars, high carbohydrate foods, practice, avoid dairy products like milk and yogurt, avoid food made with white flour [breads, crackers, etc]. Avoid starchy foods like white rice and potatoes [these quickly turn into sugar after eating], and adhere to weight control, do some sort of 30-60 minutes exercise daily, practice stress management, and get adequate sleep. These are all important in managing this illness, the most important, along with proper nutrition, is physical activity.
I assume he is type2. He should limit his carbo intake to no more than 60 per meal. It sure worked for me.
Get a meter, test before He eats, test 1 hour after, 2 hours after, if his BG goes over 140 he is eating too many carbs OR he need more medication.