Dehydration itself can make a person nauseous.
You don't have to take anything orally to treat dehydration. A bath or shower can help with hydration at times when she can't stand to take anything orally.
Dehydration isn't always as simple as a lack of fluids. It can also be impacted by a lack of electrolytes and inadequate fat.
So it may help to add salt to the bath. Table salt can be problematic for someone in frail health because of the additives. Sea salt, kosher salt or canning and pickling salt won't have those additives. You can probably find the latter two readily at a local grocery store. Good sea salt can be ordered online.
If fluids and electrolytes are insufficient and you think she may need more fat, this can also be remedied without giving her anything by mouth.
Coconut oil is high in medium chain triglycerides. These can be used directly by the body without being broken down via digestion, so the body can absorb it if it is applied topically (to the skin).
Coconut oil has a long history of being medically recommended by the medical establishment for people with serious gut issues, such as folks being treated for stomach cancer.
Don't overdo it though because it can promote nausea and diarrhea, especially when taken in large quantities. You want to do small amounts regularly so the body can handle it, not start with a tablespoon of oil at a time. That will not go happy places.
Coconut oil is a little on the sweet side, so some people don't like cooking with it because of how it impacts flavor. As an alternative, butter is a decent source of medium chain triglycerides.
Clarifying butter to make ghee can remove elements that some people don't tolerate well. This can be helpful to people in frail health as an alternative to oils they aren't tolerating well while sick.