Nightwhisper dies when you become leader. You technically "find out" a couple of days later, if you had a good relationship with her, then flowers will appear. When you click on them, your character will say "I wonder where Nightwhisper went..."
not always. she dies when you have talked to her enough for her to say, "do you think you will miss me when I'm gone? I mean, we all go someday..." she dies shortly after that
Here you go! 
Wdym? She's still here AND i recieved my nine lives
#just-murdered-firestar-and-my-bestie's-still-here