Some thoughts:
1. Mites - try eradicating with topical ivermectin drops between the shoulder blades. Common on hens which have been sitting on the nest for an extended period. Can be fatal.
2. Malnourished - need to boost nutrition. All the suggestions so far for boosting her weight have been great - good 'bulking' foods are high in protein e.g. boiled or scrambled egg yolk, sunflower seeds, dairy (yoghurt or even cheese), mealworms. I also in the past managed to accidentally make one of my greediest hens quite overweight by feeding her too much pasta and those suet-based wild bird treat blocks (softened for a few secs in the microwave). In this case, mightn't be a bad idea to try them. A good pro-biotic would also be very beneficial - try searching for Glenda L Heywood's pro-biotic mash on this board - it's a wonderful natural way to get some good bacteria inside her. Otherwise, you can buy powdered poultry pro-biotics which you can mix with feed using a wet mash technique. Again, natural yoghurt is excellent. Always make sure whatever you give her is soft and easily digested - whole grains and corn are not going to be very good right now as they take a lot of effort to digest.
3. Worms - the topical ivermectin should take care of internal parasites as well as external ones, but you may want to think about using a specific wormer if worms looks likely. Wormers are notoriously hard on them though, so don't use yet unless you have evidence of a serious worm-load... if you decide to worm her in any case purely as a preventative, do hold off until she's feeling a bit stronger.
4. You need to handle her and give a full examination - she may look okay because of her feathering, but might in fact be just skin and bone underneath. I imagine after nine weeks of being broody, she's almost emaciated. You should also feel her crop (is it getting full, is it emptying in the morning) and try and catch her doing a poop so you can see how normal it is. Look at her vent - is it normal or distended, or does it have any crustiness or poop around it? How about her abdomen? Does it feel skinny, heavy, hard, sloshy? All this may give you some clues as to whether she has an illness, or if she's just exhausted herself during that very extended period of broodiness.
Let us know what you find out and how she gets on. Good luck