How? She is already dead. Yes, true. But in an article on the Sun.co.uk, where the two manuscripts are found it says this:
The 37-year-old writer, who lives with her husband and two children in Scotland, also announced: “There’s at least one death that’s going to be horrible to write, or rather rewrite.
(http://www.thesun.co.uk/article/0,,2-2003202221,00.html)
This tells us that JKR is re-writing a death, not killing them off in the book, like with Cedric Diggory.
It would also fit in with the catagory,
Dumbledore lowered his hands and surveyed Harry through his half-moon glasses. "It is time," he said, "for me to tell you what I should have told you five years ago, Harry. Please sit down. I am going to tell you everything."
Albus, telling Harry something important. Something, he didn't know.
But haven't we already know what happened to Lily and heard about their death? Not really, because Dumbledore could go into it in more detail!
Also, in the article:
JK Rowling has said readers will learn why evil Voldemort killed Harry’s parents; the story will be “scary”;
This would tie in with the explanation of Lily's death.
The thing that Albus tells him, will;
Change his life forever
Of course finding out the story of your mothers death and finding out that thing JKR told us:
Something really important will emerge about Harry’s mum Lily;
Another point, people may have thought it would be Molly Weasley to die, because JKR had an incredible connection with her mother before she died (did she die?). But, Lily is Harry's MOTHER!
Thanks for listening.