One of the biggest problems that DESTROY our kidneys is over consumption of certain foods(mainly eating too much protei)-some people are more sensitive and therefore their kidneys wont do a good job over time,does he has any history of kidney disease in the family?(thats a big factor also)...as far as supplements,there really arent too many EFFECTIVE supplements that will make a significant difference....yeah,cranberry juice and all the other healthy "kidney herbs supplements"(whatever kidney supplements are over the counter and sell in regular vitamin stores...)-are good to use,but when it comes to a person that is cliniclly with a known kidney disfunction-i would see a GOOD nephrologist to get an opinion better than a family or an internal medicine doctor.
As far as drugs that affect the kidney negative(all the known AAS)-some are a worse some are not(Halotestin is a good example...)-they all do the same negative effect...
bottom line,tell him to look for a good nephrologist. GOOD LUCK.