In Chapter 46, he gives her a sealed note containing the explanation.
But if you ever need to fight Dementors, the secret is written here, cryptically, so that if someone doesn’t know it’s about Dementors and the Patronus Charm, they won’t know what it means...
Right, but she didn't have a reason to look at the note before she died, doesn't have the note on her person in the hospital, and even if she did she doesn't have a reason to look at the note now.
This is a new thread to discuss Eliezer Yudkowsky’s Harry Potter and the Methods of Rationality and anything related to it. This thread is intended for discussing chapter 119.
Plans for next chapter release:
There is a site dedicated to the story at hpmor.com, which is now the place to go to find the authors notes and all sorts of other goodies. AdeleneDawner has kept an archive of Author’s Notes. (This goes up to the notes for chapter 76, and is now not updating. The authors notes from chapter 77 onwards are on hpmor.com.)