You'll need to fill out a "Special Needs Form" and fax it in, well in advance of your cruise. Notify your T/A or Carnival (the question of any special needs? will be asked, or should be during the booking process). If not, notify them anyway, they'll get you a form to provide to the staff onboard info needed to accommodate whatever is needed for making the cruise more enjoyable. They can offer braille menues and daily itin for entertainment, activity schedules if needed on almost every line. Most lines also allow service animals if she has one and would like to bring it. The staff will accommodate her well although they will not provide a staff member as a guide or escort.
Send the form as early as possible so they can be sure to have anyhting she needs ordered and onboard when you get there.