I'm in a state of shock. My best friend was diagnosed with prostate cancer. I had difficulty concentrating at work today. I can only imagine what he must be going through. It feels surreal. So many emotions. Sadness and helplessness, mainly. :( I feel utterly destabilized. F_cking cancer.

There is hope. There are ways to fight cancer. Vitamin D and C boost the immune system and intermittent fasting which initiates autophagy fights cancer. There are numerous helpful videos; I prefer Dr. Berg, but your friend can find help.
The Most Important Benefit of Autophagy: Will Surprise You I wish him the best.

What you're feeling is natural, it's shocking news to hear, especially when he's been your best friend for so long. Don't give in to the worry, there is hope. I know it's hard, especially right now when things first come at him and you don't know what all is going to happen. You help more than you know just by being there for him. Remember to take care of yourself as well during this and know you have someone you can talk to as well.
A spectral rumball comes at you and turns into a *HUG*.

Dang, that sucks matterbandit. Sorry to hear about your friend's diagnose. I know these things can be tough emotionally to go through. So if you need anyone just to talk to, we're here to listen. Take care man.