If you take the broody off the nest and sit her next to some food, will she eat before she goes back to the nest?
If so, I suggest you take her off the nest and sit her next to some food as many times each day as you can. Even if it doesn't break her broodiness, it might help her eat more and gain weight (or at least stop losing weight).
For food, I would use chick starter (more protein than most other chicken foods), and maybe add some water to make it a mush-- many chickens like it better that way, so it might encourage her to eat a bit more each time.
When I've had broody hens, if I took them off the nest and sat them by the food, they would usually sit there for a little while, then sort of wake up and realize they weren't on the nest-- so they'd stand up, eat and drink, and then head back to the nest.