See if putting the ACV near her beak makes her drink, if not, then use a dropper, and try to put it on the sides of her mouth. If you don't feel comfortable giving her straight ACV, then mix a tablespoon with 1 gallon of water.
Do you know when she got sick? Are any other chickens sick? What are there living conditions like? How ventilated is there coop? (If you can post pics of the coop, that would help a lot)
My chicken was turning 3 in a couple months, but I honestly don't know how to reduce pain. Does it look like she is uncomfortable? If you can, take her inside, and just keep her on your lap, and try to see if you can get her to drink Apple Cider Vinegar if you have any.
About a month ago, I had the same thing happen too a couple of my chickens. One or two of them died, and the rest survived, and I haven't had any problems with colds and diseases since. How old is the bird?