Maybe someone can help me with advise?

my best friend SEI has a chronical health condition, which he refuses to treat. He was diagnosed about six years ago and haven't done anything to deal with it. Last year I insisted on him visiting a doctor, and he started initial diagnostic procedures, but when his doctor got transferred to another place, my friend didn't like the new one and discontinued visiting the hospital. His condition is very serious and if not treated properly will likely lead to death in 10 years or so. If treated, however, the chances are to live as long as we all do.
I live very far away from him, actually on a different continent, but I care about him deeply and I don't know what else to do to help. We know each other since we were teenagers, talk almost everyday, and are very close. I can't pretend that nothing is going on any more. Every time this issue comes up (which usually happens about every six months or so) he promises to start treatment but asks for more time. Obviously, he does not do anything about it, and years pass. He explains his behavior by fear and depression. I love him so much but I'm mad at him to an equal degree since he is basically killing himself by refusing from therapy.
I can't just silently observe him in that situation. What can I do?