I'm sorry to hear about the loss of your friend. I'm sorry that you've lost so many loved ones the this insidious disease.
We all deal with death in our very own, personal ways. I can't tell you how to mourn. Some people need to be alone. Others need to be around friends & family. You need to ask yourself what you need to do to heal your hurt.
Did you make the promise to him a moment of emotional distress? If you were not in a rational state of mind when you made the promise to him, you should consider the promise voidable. If you were in a calm, rational state when you made the promise, you should keep it.
I know that I would feel honor bound to the promise I had made to my friend, even if he were dead, but that's just me. I tend to be a very loyal person. If I had made that promise to a friend, I would think about holding a private ceremony somewhere either by myself or with a few close friends. I would go to that place that was special to him and hold the ceremony there in honor of your everlasting friendship.
May you find peace,
Ray
...they say that he got crazy once
and he tried to touch the Sun
and he lost a friend
but kept his memory ....John Denver