Your wife is fortunate to have your care and devoted concern!
I had uterine cancer over a year ago--early stage so I did not need chemo, but had numerous complications and internal injuries from the initial radical surgery that resulted in many subsequent surgeries, tests, procedures and hospital admissions. During that 6 month period I lost 35 lbs from decreased appetite and frequent fasting required for the many procedures. My husband was worried and tried hard to get me to eat more, but I just couldn't manage very much. Since I did not have the chemo as a cause, I can't exactly address your wife's reaction from my experience, or offer many suggestions, but for me it was just a matter of eating very small but frequent meals, even if it wasn't as varied as my previous normal pattern. It seemed that I lived on toast and watermelon for months.
Additionally, a weird phenomenon happened during this time: I completely lost my taste/desire for chocolate (I had always been a chocoholic before). For months, even the thought or sight of chocolate was repellent.
But gradually, very slowly, the chocolate appeal returned, as did my most of my appetite--although I have not regained the weight at all.
However, I am very pleased with that outcome, as I had wanted to lose at least 20 lbs anyway. Dropping 4 sizes is not anything I can complain about, except that I had to buy a lot of new clothes!
I don't know if your wife can afford to lose much weight, but I do know from others in my cancer support group who have had chemo and its appetite impact, that appetite changes are usually temporary and patients get past them.
So--keep trying to find ways to help her, but also take cues from what she can and wants to do so that you don't pressure her too much either. Certainly it is worth talking to her doctor or oncology nurse about it. Best of luck to you both!