I think the true argument is that dietary preference is a bit of a slippery slope. One could easily claim that they abide by a diet of only steak, truffles and lobster.
Obviously that is not feasible for a prison kitchen to fulfil. I do agree though that an effort could be made. I’m not sure if religious preference is catered to (no pork f.i.) and I could even see a point of not serving meat at all.
But the bottom line is that you can’t let the prisoner make food demands like that and be considered unethical if not fulfilled. Medically there’s not really a case here. Water and bread sounds a bit brutal, but it’s not likely that he has no choice at all, it’s also a bit of an act that his legal team will no doubt will utilise in court to claim ‘inhuman circumstances’