I'd get some electrolytes in her speedy quick, if I were you - unflavored pedialyte works. If you have some Nutri Drench or liquid vitamin B, give her some drops of that. She needs some aggressive intervention if she's going to make it. She needs food, too. Try getting her to eat some plain scrambled eggs, and maybe a little bit of some plain yogurt. A good warm soak in Epsom salt water would be very beneficial to clean her wounds and she'd absorb some minerals from the water. A 5-10 min. soak helps them feel better. I keep the chicken in the crate in a small room with a space heater after the soak, so she doesn't get chilled.
If she is very sick, the stress of getting her wet will probably kill her. She needs to be in a warm place and once she's warm enough, one could try giving warmed fluids with a tube, but only if she can still sit or stand. She should not have food until her hydration is corrected.
If I may say so, speaking as someone who has done animal rescue for over 10 years, the rescue should have instructed you not to put these new hens in with your existing flock, until 30 days passed.
The quarantine is, of course, to keep new chickens from bringing in illness, but it's also to give the new ones a chance to adapt to the new soil and various bacteria, etc. in the new environment. Even healthy chickens have their own particular "blend" of bacteria so to speak, and suddenly being exposed to a different blend can make them sick.
In the interest of the hens the rescue is trying to help, it sounds like they need to do a better job preparing the nice folks who are willing to care for the birds. I apologize if I sound too harsh or if I'm unaware of other facts that would explain the circumstances.
I agree.