How much do you want to spend? The best back protectors (it terms of combining protection and mobility) will be marketed towards people who ride motorcycles because it is designed to be worn under other clothing. Prices range from just under $100 to over $200 for the most part.
There is back-protector-only armor that straps on, but IMO the best thing to get her would be something that fits/wears like a vest. Here are two examples
A-Star Stella bionic vest
Icon Field vest
A-Star also makes vests without the elbow armor. I use the the icon. I am sure that people here use a variety of other armor.
P.S. I am in good enough shape to handle most snowboarding tumbles, but I absolutely love wearing armor that takes the brunt of most impacts... it greatly decreases the wear and tear on your body during a day of riding.