When friends die, one's own credentials change: one becomes a survivor. Graham Greene has already had biographers, one of whom has served him mightily. Yet I hope that there is room for the remembrance of a friend who knew him - not wisely, perhaps, but fairly well - on an island that was ''not his kind of place,'' but where he came season after season, year after year & where he, too, will be subsumed into the capacious story.