You said that your MIL isn't nauseous, but just doesn't feel like eating or drinking. Could she be depressed? Some people, when they find out they have cancer, go into serious depressions which, unfortunately, doesn't help them get better. If this is the case, she should discuss it with her doctors.
If not depresssion, then she must force herself to sip fluids...if she can't get a lot down at one time, she can keep water nearby and sip small amounts frequently. You may also want to look into nutritional supplements, such as Ensure, for her. If you get the supplements, they tend to taste better if they're ice cold.
Otherwise, if she keeps dehydrating, she will need to be hydrated intravenously. If this continues to occur, rather than her being hospitalized, you could try to see if your insurance would cover a visiting nurse, who could give her IV fluids when needed.