Acute renal failure can be renal, pre-renal or post renal. What is the nature of this mass in the kidney. Is it solid, is it a neoplasm, cyst etc. was there any dilatation of the kidney system. what is his creatinine, his urine output, his electrolytes. Is he on or going to require dialysis.
In my opinion there are two problems- 1. the renal failure and 2. the kidney lesion.
Renal failure may be due to reversible/ irreversible causes. If any reversible causes are identified they need to be corrected. Many factors are to be taken into account while treating the renal lesion: eg the nature (cyst tumor etc), patients general condition etc.