It might help to try and work out whether it’s:
Knows that eating and drinking are important to getting better / staying healthy, and wants to but struggling to actually do it for some reason, or
Knows that theory, but totally not interested in that any more.
Or dental / oral issues that make it painful.
The former would point towards finding ways to get what she needs into her. Drips. Fortified drinks. Very calorie dense food. Favourites.
The middle is a mental health and / or end of life care discussion.
The latter is relatively straight forward.
It’s unlikely to be feasible for you to work out which if you’re not there. So it’s a conversation to have with a family member who is.
Being in a place she wants to be is likely to help regardless. So if that’s home, can look for ways to use that as a lever and motivator. You want to go home, dad wants you home, you cant go home with a drip. Can you find a way to drink three cups of tea a day so you can go home. Or wine. Or sherry. Or orange juice. Or whatever you like.
(late additional thought - continence or mobility issues that mean is avoiding intake to prevent issues there).