Clarke
If you cannot reach him, he will still be lost. That's the nature of the Dark Side. I'm sorry. I suggest researching what you can about the Dark Side and the Light, and I suggest you come to begin training immediately. Perhaps speak to other Jedi on the matter concerning the Knights of Ren and your friend. I am only one voice with a very specific experience.
If it could help, I can tell you the name of my friend - he's here, from a time before his fall, and perhaps he might be able to tell you what it was that could have spared him.