My guess is that the rancher was suffering acute bacterial prostatitis, not prostate cancer, and the VA doctor treated him with sulfa drugs. That's just a guess. I was surprised to read that sulfa drugs (sulfonamide) are still in use, since they were the original anti-microbial drugs. But apparently they are still widely used, particularly with urinary tract infections (and hence they were probably a drug of choice with the VA urologist at the time.)
BPH is not prostatitis. BPH is just benign growth. Prostatitis is inflammation, often but not always caused by bacterial infection. That's why I'm thinking the "sulfur" was a sulfur-derived drug used to treat the infection.