Just getting in lots of what she can tolerate in many small meals is probably best,
Low blood volume is thought to be a problem with CFS. Frequent small meals may help lessen fatigue and OI by reducing the amount of blood drawn to the stomach for digestion. If I eat a large meal I can barely stay conscious so I rely on 10 small meals per day (also required to prevent hypoglycemia).
Low stomach acid and poor pancreas function might be a cause for weight loss due to poor digestion. I'm male, 5'10" and got down to 98lbs a few years ago. It didn't matter what I ate, I just couldn't gain weight. I then found that digestive enzymes helped reduce fatigue after eating, and Betaine HCL (with pepsin) helped even more. I now average 115-120 lbs; still low, but better than lower.
If your wife is tired after eating a small meal, especially a high protein meal, then she may have low stomach acid. If she's tired after eating a large meal then try a meal 1/6 the size of normal to see if the fatigue lessens.