This is a nifty find, and might make a little bump in the popularity of an acknowledged classic, but faking it would hardly be worth risking her career & reputation.

The granddaughter's job is managing L'Engle's estate.

Plus, it seems to me that a fake would be quite possible to expose -- the age of the paper/ink, uniqueness of typewriter characters etc.. It's one page of an entire paper draft of the novel, typewritten (presumably on a single machine) 50 or so years ago.