you can buy some cheap frozen food for her and ask her to cook her own meals once in a while (not always).
I buy some cheap frozen 秋刀鱼 4 fishes for HKD10 dollar only, 2 fishes each meal then on average is only 5 dollar
I also buy frozen chicken legs (on average 10 dollar for one), and frozen pork chops. Sometimes the wet market has fresh fishes, like 5 fishes for 15 dollars, I will buy for her too.
In addition, you can buy eggs, sausages, frozen crab sticks---these are all not expensive.
When we eat very expensive food for dinner (like tiger shrimps, crabs, etc), i'll ask her to cook some of the above frozen food for herself. Sometimes when we have nothing left, she can also eat that.
Remember that their food allowance is about 1000 dollar for 26 days. so on average, each day the food should count no less than 30-35 dollar (of course you'll have to count the utilities, soy sauces, etc). So I think for dinner/lunch, meats for less than 10 dollar should be ok---this means she can have 4 frozen fishes or a chicken leg or 6 eggs etc plus some vegetables...that should be more than enough for her.