Dude, I know Missouri is right buy Oklahoma. Never been there in the summer, but if it's anything like it is here, I would do whatever it takes to stay cool. 110 in the summer is terrible, and we have a humid heat, which makes it seem about 120. I have seen plenty of people get heat exhaustion at horse shows, and it's dangerous to let your self over heat like that, especially if you are at an outdoor show..
I really wouldn't blame her for wanting to dress lighter.
It depends on how 'big' the shows is. If it is of any importance, you are sacrificing your 'look' by not having the cuffs showing at your wrists. I use to wear a sleeveless shirt, but once my coach told me that I HAD to have a long sleeve shirt on for a regional competition, I never wore the sleeveless again, even at the smaller shows. I figured, if I am going to spend the time grooming, braiding, and put out a good performance, I don't want my 'look' cheapened by not having long sleeves. Plus, it did solve the problem of "no jackets required" on the hottest days. Nothing worse than them announcing no jackets required, and then HAVING to wear the jacket, because you were wearing a dickey/sleeveless shirt on underneath!