How does alcohol actually affect a person's kidneys who has diabetes?

Via filtration: The role of alcohol has dual properties to someone who has diabetes. Ironically alcohol alone has a lowering effect on blood glucose since its largely metabolized by the liver. It can halt production of glucose by the liver. On the other hand the increased carb load that is associated with e.G beer contribute to increased blood glucose which gets filter by the kidneys.

Depends: Depends on how serious the diabetes and how serious the drinking. In type 1 diabetes it can be more dangerous, and can precipitate diabetic ketoacidosis if you drink too much. It also can lower blood sugar, so if you drink and take insulin, you can get serious hypoglycemia with passing out and even seizures. In type 2 dm, alcohol is high calorie, almost as much as fat, so that is also bad.