On the value of punctuality.

If anyone sees Miss Abbott, please feel free to remind her that Emily Post recommends guests "should arrive at or shortly after (usually only fifteen minutes) the time stated in the invitation. Do not, however, arrive early."

Or should call if they intend to cancel.

(from: http://www.emilypost.com/social-life/hosts-and-guests/466-party-etiquette-tips-for-hosts-and-guests)

Edit: The situation has been clarified.