I'd give her an epsom salt bath but I have already traumatized her. Likely as not, I'm the reason she is dying.
As to was she a healthy hen? The following is the long version of the short answer which is "I don't know if she was healthy." Brownie (the currently dying/sick hen) was adopted in 2014 and had two sisters, Clownie and Downie, both of whom died last year, one of confirmed cancer, one out of nowhere. She just up and died. All three are golden-buff red sex-linked. Note: All my girls are on organic feed. Free range. Get healthy treats. Having already lost three other chickens to cancer over the course of my 5 years of chicken stewarding, I was and am at a loss. But I digress.
Back to Brownie. Last year, a bit before her sisters went the way, she lost all her underneath feathers. Not a molt. She just walked around with a bright red belly and no feathers under her. An occasional down day but, as soon as I approached, up she'd jump to chase a grasshopper. I was worried about her featherlessness as winter approached and so, when I took another hen, Peaches (cream leg bar) who is a whole other story in to the vet, I took Brownie, too. He took a scraping of Brownie's tummy and it came back with a vitamin A deficiency. The vet thought that the cancer was a result of the same. I had to put Peaches down that day--cancer. And two days later, had to put Clownie down. Cancer. Not a good week.
Brownie eventually grew her feathers back to fluff and order and continued to lay her quirky eggs. (Huge. Oblong.) Until a week ago, when she started sitting all huddled and uncomfortable looking. After a few days, she looked ready to die so (fool that I am) I put her in a box, all cozy with hay, and brought her into the house for overnight so she'd be warm. The next morning, I went to check on her, the box was covered with bugs. The vet confirmed northern mites.
Needless to say, the box and chicken were whisked outside. After vacuuming the house in a panic of mite infestation, I spent the rest of the day cleaning the coop, and looking up natural mite deterrent/destruction. I live off grid on a farm with a just built pond and so the insecticide I naively bought -- that says on the label is death and destruction to frogs and such--got returned in favor of less awful stuff.)
I bathed Brownie in orange oil and water. I sprayed the coop with neem oil and the orange oil stuff. I spread Diatametous earth everywhere (and breathed it in but at that point I was trying to save my chicken.) Gave Brownie a dust bath in DE and wood ash. That she didn't seem to like as much as the warm bath. Then I spent an hour preening her with a hair dryer drying her. She seemed to like the warmth.
Yesterday, I recleaned the coop, gave her another dust bath--that I think brought her to her edge. I then read about garlic spray. It kills mites within 24 hours!? Had I only known. I made that but by then Brownie was where she is now: at death's door. Worse, she had a bloodied comb. So back into a new box to keep her separated from her sisters and roo. I feel awful as I think I killed her through the trauma of trying to save her. I'm ready to toss in the towel with chickens. And still I hope she returns to her perky self somehow. And I look at her sisters. And know I'll keep them. But what to do? She's suffering yet overtime I try to help, I only make matters worse.
Sorry for going on so long. Maybe, as Rebrascora said, she was already a sick chicken and the mites took her over the edge, not my tender loving care. . . . :-(