I agree with Eddie, the best you can sensibly do is to provide warmth, peace and quiet, food and drink if she wants it, and a bit of comfort. Don't put her back with the others, the shock of being attacked and the cold weather will probably be fatal. It will be sad if she doesn't make it, but prolapse is common with hybrids who have been laying very intensively in commercial conditions, it is a serious condition, and its not easy to treat or usually successful longterm. Maybe give it a day or so and see if there is any improvement. There is very little a vet could do over and above what you can do for her yourself in the next few days, ie keep her clean, warm and comfortable, and hope it stays back in. A vet might prescribe antibiotics in case infection sets in, but will probably charge you upwards of £50 for a consultation and prescription, and then she still might not recover, or only for a little while until it recurs. The probability is that she had this, or was developing it, when you got her. It must be very stressful for hens to go through the process of being bundled out of their cages and all mixed up together and then rehomed, and stress causes all sorts of problems at the best of times. Once they have settled down the rest will be OK I expect, but you may need to be prepared to make a hard decision about this girl, I'm afraid. Exbatts are not always the most straightforward of hens to take on as your first birds, because they do have special needs and problems, as you already knew. Good luck with her, it may be that she will pull round, I do hope so.