Good for your daughter for still following her passion. Can you watch her from the car mom?
There are a few options out there for her, to make it a bit more pleasant. Army Issue Horse Guards Waterproof Riding Cape
(it's a UK site, but I didn't look to see if they ship abroad).
Barbour also sells long wax jackets that have an attachment to go over your legs and the horses rump, but they are expensive. I mention it, as you sometimes see them cheaper on ebay
I have a Campbell Cooper, which has leg straps on the inside and the attachment for the horses rear end. Home - Wax Jackets by Campbell Cooper
Sorry all the links are British - I am, and so more familiar with where to get the products there, I am sure that someone else will be able to help you find them here, if you are interested.