Sorry for you loss, I hope I can help a bit. You best bet is probably going to be explaining the situation to their family and ask their help in promoting a new clan leader. Now that's a hard ask and I don't envy the person that reaches our to them, but his friends are tying to honor his memory and they may be willing to help with that. If they cannot access his account you may need to work with Microsoft/Sony/Steam (with the family's help) to regain access to his account. All of these companies should have a process in place for when an account owner passes on. I tried for a long while to get control of my original clan or to at least get ownership passed to someone who would manage things. Bungie never stepped in, and they never will. Unfortunately this has held true for many people in your situation over the years.