Hiya

Nice to meet you!

From what I can see you have a few issues going on. Her feathers suggest parasites. She looks thin and in poor condition could be internal parasites too.
Have you wormed your birds?
If she has mites or live you’ll need to treat all your birds AND their coop after replacing all the bedding.
Worms and parasites will need follow up treatments to ensure you stop the life cycle. Eggs/pupa/adults laying eggs.
Mites feed at night and they can drain a chicken to the point of anemia.
Then you have the reintroduction process. Since she’s been away from the others you’ll need to slowly reintroduce her. This might help her acclimate to the temperature. You could wire off a pen within the pen. Or use a dog crate with a tarp or horse blanket over it. Taking her out for longer periods each day. Until you release her right at bedtime. See if she’ll go to roost for the night with the others. You’d need to watch the next morning for possible issues. You should watch until things go smoothly or perhaps more time in her pen/crate.
Now would probably be a good time to give each of your birds a complete exam. Head to toe right down to the skin. All vents and legs. Access what you’ve really got going on then form a plan of action and treatment. Best wishes