The first step should have been to explain it to him in a way he could understand. If he hasn’t understood (and clearly, if this is what he was tweeting, he either wasn’t given any explanation or the explanation was poor), then it’s the HCP responsibility.
Or possibly he was told but couldn’t retain the information as he was distressed from the stroke and from feeling thirsty. The fact that he specifically mentioned a litre of water sounds like this was mandated for a medical reason.
My mum is in hospital right now and, although she is ok mentally (broken bone), she is tired and and struggles to retain what the HCPs tell her, so we ring the nurses if we want to check things.