how often to give green stuff

I have discovered that my girls love Kale, my cavolo nero has been well and truly munched by caterpillars so the chooks are regularly getting abunch of leaves hung in the run, they go mad for it. How often should I give it, is every day too much? I have been giving it every other day but they have had it the last 2/3 days in a row. Mrs Pepperpot has a mucky vent this am but have just observed her doing a nice "normal" poop. She is the one that is more nervous than the others and I can't get near her. She also has a different cluck to the others, almost like she has a sore throat. I think she is laying, can't be sure as we are getting 3 eggs from the 4 girls. Should I worry about her, she is eating and drinking.

Yes, give them greens every day. I am always amazed how much they can eat. They also like cabbage, lettuce, spinach, swiss chard, radish tops, cauliflower etc. Its worth growing an over winter crop that you can feed to them.

They love their greens. I give mine weeds as well such as chickweed, groundsel and docks
Beetroot leaves are a fave with my girls.  I grow brassicas for them at home so I've always got fresh supplies as it is quite costly when you're feeding them on a daily basis. 

It sounds like your hens are in Hen Heaven.