Frozen feet

My dd had another horse riding lesson yesterday, but it was around 38 degrees the entire 1 1/2 hours she was there. She normally wears one pair of riding socks (about the thickness of dress socks) and rubber boots. Her feet were cold within minutes after arrival. She said it hurt really badly when she had to jump down from the horse at the end of the lesson and every time she needed to push her heels down while riding. Does anyone have any ideas about how to keep her feet warm during riding lessons? We only live 10 minutes from the stables. I am wondering if they sell socks with some kind of gel that can be heated up, kind of like those hand-warmers they sell at Walmart.

I had thought I would be cold as well sitting in the car with the heater off, but the sun shines straight through the windshield and turns the inside of the car into a nice toasty place to sit.