These birds are in an outdoor aviary so they should be getting enough sunlight to make their own vitamin D.
Greens in general are not a great source of calcium. They contain calcium of course, but they also contain a lot of antinutrients that interfere with the absorption of this calcium. That's just the nature of plant foods.
Cuttlebone is a good source of calcium, as long as the bird actually eats it. It's mostly calcium carbonate, which is digested and absorbed pretty easily. You might want to consider making some mineral grit available too. I know the internet says grit is evil but I think this is wrong. It's part of the wild diet of cockatiels, so nature has obviously equipped them to deal with it.
Hens are supposed to crave calcium when they need it (it's called a specific appetite) but I'm not convinced that all of them have gotten the memo. Maybe this hen isn't eating enough calcium, or maybe she just needs more than most hens do. Every bird is an individual and some of them don't follow the rules.
There are calcium supplements for birds that you can buy and mix with the water or food, like this one: http://www.morningbirdproducts.com/p...lciumplus.html
You have to be cautious with calcium supplements however, because it's possible to overdose. Too much calcium is as bad as too little.