How does diabetes affect a person who has one kidney?

Same like otherwise, However, persons who have 2 kidneys have more tissue which means that the loss of function is expected to be faster in persons with only one kindey if diabetes sets into the kidney. If diabetes develops late in life then the worry is not as much since it is a function of the duration of diabetes. Usually, but not always, a 15 years of diabetes yields involvement of the kidneys.
No different than.. If they have 2 kidneys when sugars damage kidneys, it affects both at same time if have 2 kidneys or the 1 kidney if only have 1 kidney.