Sounds to me like a respiratory disease, but I can't be sure. It would be better next time just for you to take the bird inside and give her some quiet place to rest where it is cool and dark. I wouldn't advise putting them in water for the purpose of cooling either. I also wouldn't think that your rooster broke her neck. He was probably just taking advantage of her, which means that she might have already been displaying weakness long before she got to the point where you noticed her. It's not always easy to tell when chickens aren't feeling well. If you watch for 20 minutes or so, you might notice inactivity in one bird. If all the other birds are running around, pecking, drinking, etc., and you have one bird just sitting there all fluffed up, it can be a sign. Also, you might've noticed if she was gaping her mouth open on occasion? That would be a respiratory problem.
Also, from my own experience, I had one bird die mysteriously once just like yours. When I found her, she was still alive, but having trouble breathing. She died shortly afterward. I let my chickens out of their coop and went inside for about 20 minutes. They all seemed totally fine. When I came back outside, she was lying on her side in a dark corner of the yard. Upon very close inspection, I noticed she had a puncture wound just up under her wing. I've never been 100% sure, but I think it was a hawk that just flew off right as I was walking back outside. This can happen too, and I almost completely missed the injury myself, so you might've had a predator attack... I can tell you that after that day, I caught a LOT of hawks in my yard harassing my chickens.
