Those don't sound like prostate cancer symptoms. Typically there are no symptoms whatsoever when it's localised, and bone pain if it's spread into the bones. But flulike symptoms - no. As Chris said, it's extremely common for prostate cancer to be found when something else is being looked at - that's exactly what happened to me.
When you say your friend has been diagnosed, do you mean he's gone through all the diagnostic tests - MRI scan, biopsy, bone scan - and been given a final staging, or has he just been told that he may have prostate cancer?