Is it good for a person with diabetes (type 2) to drink diet soda?

Better than non-diet: If it is your habit to drink non-diet soda then the diet variety is clearly better. Non-diet soda typically has high amounts of sugar that leads to unwanted weight gain and makes control of blood sugar more difficult.

Better than regular: Diet soda is better than regular soda, especially for a diabetic patient. However, there are much healthier options than soda such as water!