ETA: If you need help empathizing, think back to when you were pregnant. It's definitely not the same, but it was likely a time in your life when your body felt like it was not your own, in a way that was not simple or easy to explain or even comprehend. Being able to relate to that part might help you not to be annoyed with her, which can easily happen as a caregiver.
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That is so tricky. I work at a cancer hospital, and I have assisted in the care of quite a few loved ones dealing with this.
Right now my uncle has stomach cancer and had/has physical therapy to help to keep him moving.
One thing that I've found helpful is to have things around that aren't full of flavor on their own and can be dressed up to preference. Jicama is just cool and crisp with no big flavor. It feels good going in. It can go with anything or be eaten alone. It's good to have on standby for grazing. (I ate a lot of it when I was pregnant, when I wanted to graze but didn't have a taste for anything.) Potatoes are good to keep around. They are filling and stick to the ribs and are bland enough for a weak stomach or palate but can be flavored to suit any craving. For some of "my" patients, I have bagged and frozen one-serving portions of potato soup just for them to keep on hand, especially after treatment. You can slice or cube a potato and keep it in a closed container of water in the refrigerator. Cook some on the stove (quick boil) and flavor to her immediate liking. If she does not have the energy to chew, mash them for her, and just continue to emphasize the importance of getting in just a little at a time.
No dairy. (This will promote mucus formation, and that's a bad idea when her lungs aren't healthy and she's lying down a lot and not necessarily strong enough to cough if she needs to.)
A juicer is a good idea, as well. Drinking her nutrition will get it through her body more quickly, to where it needs to go without using up her body's energy on processing the food (breaking it down for use).
Look into the resources at the place where she's receiving her treatment. They should be able to steer you toward nutritionists and therapists who have seen this before. A support group for you might not be a bad idea. If you don't find what you need in your area, you might be able to find something here: http://www.mdanderson.org/.