You did not mention which drug caused your wifes tubulointerstitial nephritis and how severe it is. Depending on that different drugs cause varying degrees of severity and kidney damage. However, I am assuming the offending drug has been discontinued since it may cause further progression of the disease if it is not.
Not much can be offered to reverse the damage her kidneys have already sustained barring certain circumstances e.g. an allergic reaction to drug may revert following the discontinuation of the drug and in certain cases a short course of steroids may speed the recovery. Again, steroids have been tried in various cases of drug TI nephritis with varying success. On the other extreme, TI nephritis caused by certain weight loss medicines contained in certain chinese herbs result in relentlessly progressive kidney failure.
Other suggestions include - strict control of blood pressure, blood sugars (if she is a diabetic).