Not necessarily. Studies show that 10% of domestic birds carry 90% of the parasites. They will go after those with weaker immunity first. Or rather, the ones with weaker immunity will succumb and show symptoms first.If she has mites or lice would all of his chickens looked like this?
When it comes to integrating birds, mites and lice aren't even the scary things your have to worry about your birds catching. But rather Marek's, infectious coryza or bronchitis, Newcastle disease, flow pox or even avian influenza. To me, your other birds are already exposed. Don't think I would separate again.

I got girls once that came from a place with no grass. It took them a couple days to really start enjoying the new environment. But when they did, it was really special! I could tell they were so much happier.

I would consider looking at directions on how to make a chicken saddle. And use that to improvise some warmth for her. I'm not surprised a regular t shirt didn't cooperate.
http://youre-sew-crafty.blogspot.com/2013/10/chicken-jacket.html
I thought this pic could be helpful to figure out how to make your own and what it should look like. It looks like a simple thing to copy even using regular material....
