Sorry for answering very late, but this is important information nonetheless for anyone with prostate cancer (or pretty much any other cancer). If your friend isn't taking vitamin D, I think he should. If he's taking the RDA of vitamin D, he gets too little. The RDA is too low based on current research and leaves people at risk of many chronic illnesses. Some rare diseases (at least some lung cancers, Non-Hodgkin's lymphoma and sarcoidosis) may cause problems with vitamin D so discussion with the doctor is essential at least in those cases.

In a study with prostate cancer patients in Norway, people with low vitamin D level were 3 times more likely to die of the cancer during the study than others.

Vitamin D can also lower PSA in some prostate cancer patients.
http://news.bbc.co.uk/2/hi/health/8017323. stm

Here's a quote from a recent article in Annals of Epidemiology (a peer-reviewed scientific journal):
"These findings provide strong evidence that vitamin D status plays an important role in controlling the outcome of cancer. Support for the UVB-vitamin D-cancer theory is now scientifically strong enough to warrant use of vitamin D in cancer prevention, and as a component of treatment."

One of the best sources of vitamin D information is Vitamin D council.
http://www.vitamindcouncil.org/

Divide nmol/l with 2.5 to get ng/ml.

If patients had vitamin D levels of 50-80 nmol/l, they were only 33% as likely to die from cancer as patients with less than 50 nmol/l. With levels greater than 80 nmol/l, patients were only 16% as likely to die as patients with less than 50 nmol/l.