"But who'd be that desperate?" he wondered aloud. "If you're going to be cursed forever, death's better, isn't it?"
"It is," Firenze agreed, "unless all you need is to stay alive long enough to drink something else –– something that will bring you back to full strength and power –– something that will mean you can never die. Mr. Potter, do you know what is hidden in the school at this very moment?"
"The Sorcerer's Stone! Of course –– the Elixir of Life! But I don't understand who ––"
"Can you think of nobody who has waited many years to return to power, who has clung to life, awaiting their chance?" (Harry Potter and the Sorcerer's Stone)
I guess ‘their’ does not have a plural meaning here but it is a substitute for his or her. Is this right?