Aloha. YES. Separate her immediately. Keep her comfortable and away from drafts with plenty of water and feed. You have two options here. You can give her a natural antibiotic called Silver available at any health food store. It is inexpensive. Give her a dropperful twice a day. Or you can get a broad-spectrum tetracycline from any feed store. It is inexpensive and keeps forever. But when you make a batch (the smallest batch I've been able to make is 1 gallon which is 2 and 1/2 teaspoons to a gallon of water) it is only good for 24 hours. So be sure and whip up a fresh batch each day. If she is eating and drinking, replace her water with the tetracycline solution. If she is too sick to drink, gently administer a dropperful a couple times a day. She should show improvement in just a day or so. Use either medicine for a week.
Now...you should also dose the other pullet. You can keep the two together if both are being treated. If you have released either of them into your flock, dose EVERYONE. But it sounds like you were smart and kept them separate. Keep everything clean and be patient. Both will pull through. Do not buy from that person again. It was probably only ignorance, but you want to purchase from knowledgable folks. I would definitely let the seller know it is not dried poop. S/he may be unaware.
Sounds like you are really on top of things. All should be fine in about a week. Do NOT stop just because your baby perks up. Go a week. j mccabe
