For those that don't live in an area overrun with a commercial chicken houses (as we are) I can tell you what would happen if she took it back. It would be killed on the spot. It was out of the control of the commercial grower, albeit for only a short while. It's been exposed to other (non-commercial) birds. No commercial farmer is going to take a chance on putting that one bird back in the broiler houses.
Chickens fall off the trucks or escape, either going into the grower houses or coming out of them (on their way to be processed). The poultry industry considers those birds acceptable losses. That's not taking into account all the chicks that they cull at various points in the growing process. They do at least three culls during the 6 to 8 week period.
To the OP - I think she would be warm enough in a draft free coop, but your solution of putting her out during the day and bringing her in at night works too. Just make sure she gets plenty of time outside, moving around, during the day.
My rescued broiler (Cobb on Cobb) chicks, which are a different creature altogether than the cornish rocks, are now 23+ weeks old and every one of them is laying. They've done fine in a mixed flock with layer birds that they were raised with.
I started out with 19. The commercial farmer that gave them to me told me to just "pinch the head off" of any I didn't want to keep. That's how much the commercial industry as a whole cares about losing a few birds here and there.