They will get it under control, but it takes time sometimes. No more fluids is not an option...unless they prefer death. The human body CANNOT function without fluid and electrolytes. We had weeks of severe diarrhea with one of the chemo regimens, Nothing would stop it.
Now I start medicating heavily at first sign of it (loose), and stay on top of it and have found we were able to manage it. Add fluid to compensate for the loss that goes with violent diarrhea, and they could be almost normal.
I posted recently about how little discussion there is here about fluid, and how crucial it is to keeping the patient in good condition. If you can keep up with it, the side effects tend to be less intense. The fluid is also helping to remove the toxins and dead cells that are making them sick.