For example, she has expressed the idea that if she doesn't drink, she will die. If she honestly believes that restricting herself to an ordinary level of water consumption will result in her death, is it a surprise that she is reluctant to drink less water?

From the information which you have posted, I do not know if she currently believes that she will die if she drinks less water or if she no longer believes this. However, she could easily have other fears and beliefs as well such as "If I don't drink I will feel weak", "If I don't drink I will feel anxious", "If I don't drink I will get fat", "If I don't drink toxins will build up in my body", etc.

It may be important to find out what she thinks or fears will happen if she only drinks ordinary amounts of water. She will be quite reluctant to limit her drinking if she honestly thinks that doing so will lead to disaster.