Advance Directive

So I am writing a paper on the use of advance directive in the case of Alzheimer's Disease. The situation goes something like:

Before she retired, Vivian was highly respected professor of English literature, specializing in the intellectually-challenging metaphysical poetry of John Donne. She was an intellectual powerhouse, and her whole self-conception was tied up in the dignity she finds in the life of the mind. For that reason, she wrote a very careful advance directive that specified that if she becomes demented enough that she can no longer recognize her family, then she should not be given any medical treatments, not even antibiotics to treat her routine infections. She is now suffering from Alzheimer's Disease, and she has become so demented that she no longer recognizes her daughter, Evelyn. Yet, in her demented state, she seems perfectly content with her life. Indeed, Evelyn thinks Vivian has never been happier in a certain way: she smiles at simple things and laughs more. However, Vivian now has a serious, but treatable infection.

Should Evelyn be permitted to ignore the advance directive and authorize treatment for the infection? Why or why not? Please give me your reasoning behind it