S.B.
This is a sticky situation.
I'm wondering though if part of your daughter's misery is due to the fact that she's never been around anyone who smokes and smoking "non-stop in the car" might have been a bit exaggerated.
I'm not saying your daughter was making it up or anything, but I have a daughter and my parents smoked. They didn't smoke in her immediate presence and it never bothered her until she hit about 11, 12, 13 and she would freak when they were smoking outside. She would claim to be able to smell it coming into the house or that even outside the smell would get on her. They were outside in their own yard and that wasn't good enough for her and she, frankly, was a bit of a brat about it.
Then, guess who started smoking when she turned 18. And, she knew full well all the fits she threw and how mean she was so she wouldn't smoke around me. She didn't want me to know.
She only smoked for a couple years and quit completely, but the hipocracy wasn't lost on her.
I think you should quit beating yourself up over this and tell your daughter that maybe she should just talk to Grandma and Grandpa and nicely say that she's not used to being around cigarettes and the smoke kind of bothers her.
You said yourself that you didn't even think about the cigarette issue...maybe your parents didn't really think about it much either. It doesn't sound like your daughter goes and stays with them all the time and these things have been worked out.
I don't think it has to be a big confrontation.
I'm not sure that you have to go running to get her or make a big stink about her coming home early and never going back.
Your parents might really be enjoying having her and if she mentions that it bothers her in a way that isn't disrespectful or snotty (like my daughter did), they may be willing to make some compromises for her visit.
They may be more willing having it come from her than you ripping into them about what they do in their own house.
If they get totally mean to her about it, that's a different story, but I would give her a chance to go that route first.
My sister and I got to go and spend summers with our grandparents and extended family. Some of them were smokers. As kids, I guess we were too busy running around outside and swimming and climbing trees, we wouldn't have wanted to come home over the smoking issue. We never thought much of it.
Like I said, if your daughter doesn't even know anyone who smokes, it's bound to bother her just by virtue of someone smoking no matter how close in proximity they are.
And, there is a possibility that your parents will be willing to smoke when they stop for gas or at a rest stop as opposed to in the car. They may be willing for the time she's there to smoke out on the porch instead of in the house. I think they'd be more willing to make those compromises if a huge deal isn't made of it.
That's just my opinion.
If you make her come home early or go get her or demand they don't smoke around her, the stubborn gates really will go up.
Try the softer approach.
I hope it works out and your daughter can enjoy the rest of her visit.