My neighbor absolutely swore by almonds. Sounds odd but it worked for her. The protein was good for her - better than the typical saltines. She liked the lightly salted ones and just kept them at hand, in the car, etc., and just ate 1 about every 20 minutes.
The additional problem with morning sickness is when it degenerates into such severity that the actual decline in sugars & salts results in endless dry heaves. So when I worked in a clinic we often had to run an IV just to get the woman back up to the level where they could eat something. So, the being sick just makes you sicker - if that makes sense. I've often recommended a particular sports drink that is patented (therefore safe) and lacking artificial dyes. The sugar in it is real fructose (NOT high fructose corn syrup like you find in most ginger ales). It is highly absorbable in just a few minutes so often, if the woman can keep it down for just a little bit, she gets the benefit. I've also recommended a comprehensive and balanced general nutrition formula that, again, is highly absorbable - so what little a woman absorbs even if she does get sick at least provides some benefit. We find a lot of women can tolerate that (it's also used by cancer patients who are often nauseated from chemo) and it's additionally beneficial because a lot of morning-sick women throw up their prenatal vitamins (which themselves can cause digestive upset).
The chicken soup is okay if she's in the mood. But there's not much you can give her that's so nutrient-laden without being hard to digest. The mashed potatoes worked for you, but there's not a lot of nutrition in them. The best thing would be for her to eat something that calms her stomach enough that she can eat other things little by little.
Also I'm sure she knows but you can remind her to eat small amounts very frequently. She's got to keep her protein level up and stay hydrated.
I'd imagine that taking care of her other kids, providing meals for the family, maybe doing errands (if she's sick or dizzy, she shouldn't drive much), letting her rest, etc. would be even more beneficial. Maybe you can organize some neighbors or friends to just pitch in without her having to actually make all the decisions? Maybe that's what she means when she says she doesn't need anything - she just isn't up to deciding, or she feels that nothing will work, or both.