URGENT: Need help with a Cornish Cross

Hi! My high school got 60 chicks near the end of September. We ordered 15 Rhode Island Reds, 15 Anconas, 15 Delawares, and 15 Australorps. Or so we thought. The hatchery accidentally sent us a Cornish Cross in place of one of the Delawares. We decided to keep her with our flock of now 55 birds, but she's having A LOT of problems. She's had sour crop 4 times in the past 3 months and every time it gets really bad (size of an apple, stretched skin, visible gas bubbles, so huge she kicks it when she walks) and no matter how many times we treat it (apple cider vinegar treatment continues for a day or two after it goes away just to be sure) it always comes back. It's gotten to the point where we're considering mixing apple cider vinegar with the water supply in the coop. Our hens live in a giant coop/run enclosure, they get fed fresh vegetables and food scraps from our garden, and they always have clean water. The hen looks awful. All of the other hens are healthy and our two roosters are gorgeous. The meat hen looks like someone dumped her in dirty water. All her feathers are dirty and stuck together. She has NO feathers on her underside (from her vent down, under her wings, under her crop), she has trouble jumping into their coop (like 2 feet off the ground) and she can't fly or run at all. She also doesn't perch and sits in the poop. What do I do? I don't want to kill her because we all love her, but I'm not sure she'll live a good life. I've attached some pics, one of her in the coop (with the other birds roosting above), a picture of our coop/run area, a picture of some of our other birds and our rooster (to show that we have a healthy flock), a picture of the cuddle puddle with the entrance of the coop behind them, and a picture of our hens crowding the gate when I brought them food (for size comparison. Please help!