My daughter has 27 voices. 5 of them are recent. She talks to me about them. What I have noticed with her is that when she is under more stress. the voices get louder. Recently, during a time that was the most stress she's been under, she acquired the 5 new voices. She is on medication, but that only seems to quiet them some and not be so violent. When the voices first started with her, they acted like protectors and would tell her how to "protect" herself. But eventually they became violent and started telling her to cut herself or hurt herself...or even someone else. Eventually they started to tell her to kill herself. Once she was on the right combination of medications, she started to do better. I don't know if she will ever not have voices...even with medication, but at least they are more under control now. My daughter feels best when she is able to talk to me about it.
Your friend is blessed to have you. Keep listening to him and being there for him. That is a wonderful gift to give him.