His life belongs to him and him alone. If he decides to depart this mortal coil by refusing treatment (which may be painful or ineffective, considering his condition) then that decision belongs only to him.
Your duty is to accept that decision and console both him and his survivors. Sit at his bedside before he dies and tell him you understand, and that he's been a terrific friend whom you will miss mightily, and hope to see again someday.
As to his next-of-kin, the same goes. Dying is a private business, and the decisions come deep within us. You can offer all the support you choose, but I assure you, however well-intentioned your advice on the siuation might be (and I am certain it is) it is unwelcome.
My condolences to you. If you feel the need, most hospices (It's a yellow pages heading) offer grief counseling for free.