In the time since the Baudelaire parents' death, most of the Baudelaire orphans' friends had fallen by the wayside, an expression wich here means "they stopped calling, writing, and stopping by to see any of the Baudelaires, making them lonely". You and I, of course, would never do this to any of our grieving acquaintances, but it is a sad truth that when someone has lost a loved one, friends sometimes avoid the person, just when the presence of friends is most needed.