The problem here is that they probably came to you with CRD (mycoplasma.) Any chick or chicken you bring into your place from now on will be exposed to her. She may have already exposed the other chickens, since it can travel through the air and in their droppings, and on your shoes and hands. It's up to you if you think the others are exposed already, just keep her. Even though many people have it in their flocks and just treat their birds when they have an out break, you need to think about what is best. I have never dealt with it yet, and I know how hard it is to give up a chicken when we become attached.