Oranges are very healthy for diabetics and anyone else because they're high in Vitamin C and other health benefits.
For a diabetic who needs to watch their sugar intake the diabetic can safely eat one orange per day and should avoid anymore than one orange per day.
If the diabetic eats more than one orange per day then it could cause a spike in the diabetics blood sugar but as a diabetic eating one orange per day is safe.
Just keep check on the blood sugar levels if you're diabetic and eating fruits such as oranges or any other fruits that contain natural sugars.