It's best for them to go inside so they aren't exposed to predators and the longer they roost outside, the more they will want to do that. How cold do you get? Guineapeeps is in Iowa mentioned her birds survived being out in 20 degrees below during a snow storm. Not ideal, but they are pretty hardy. Maybe try putting food in the coop area. Not sure how that will work with your other birds. When we changed them over from an outdoor coop / run to a small barn building, it took them about a month and a half. I kept the doors opened, kept food in there and then they just started exploring. Once they are comfortable and have been with you a bit longer, you can try keeping them inside the coop for them to get used to it as well. A lot is trial and error, but you've come a long way with them in a short amount of time and Cher will come around quickly. You'll see - they'll be like different birds in just a few weeks.