It's amazing what a little friendship can do. Seeing the other birds doing natural stuff also will help her to feel more happy and productive I am sure! Good for you for volunteering! I used to volunteer in the rehab department at the zoo. We had more raptors and wild birds (bob cats & stuff) since it was only for Ca native animals at the CALM, Ca Living Museum. A wonderful experience.I volunteer at a poultry rescue and more often than you think I see depressed birds and to me that sounds like what you have...
A few weeks ago I had a small production red hen come in that was not eating no matter where you put her or what you gave her, I took her under my personal care and she started to perk up when she was with my young super friendly Cochin hen. After 3 days she started to eat and I was able to get her in with the the other chickens at the rescue
Introducing chickens is not always the easiest thing to do. And you will want to make sure you are getting a pullet not a roo.Okay I definitely going to get another chicken, but I have no idea what type of chicken feed to get her
You may have to adjust to her sleeping outside with her friend, I promise 2 house chickens although fun won't be that much fun. It will free you and her both up to do more enjoyable stuff because she is 100% dependent on you for her social interactions right now. Also I sincerely recommend quarantine when bringing in a new bird so you don't end up with a mystery illness as symptoms might not be showing yet and they need time to develop some immunity to the bacteria in your ground as well.
If you need a specific suggestion, Purina Flock Raiser is what I am currently using. I don't claim it to be the best but it does appear to be acceptable. Good luck!
You will want to keep any extra seed, cracked corn, fruit and veg, mealworms (a chicken fave) to under 10% of the total diet in order to not dilute the nutrient load of the feed.