They are going to charge him if they can charge him. This involved a murder. If he was part of it, his cooperation will not matter.
Let me be clear, I have never been arrested..... but I would never talk to police about anything that could actually involve me. Im a licensed gun carrier, and I had an incident once where I made a report. The way the police questioned me about the incident made it clear they were trying to see if they could charge me with something.
Lesson learned. What took place, what the police say took place, and what they might charge you with could all be very different. Let the police do what they are going to do without your help. BUt hey, thats why KJ has a lawyer for. Let him advise him on what he should be doing. Which likely means cooperating but always with his attorney present. Talking without his attorney present is FOOLISH.