+1 but if she has a previous injury it would be good to wear something. I bought mine for rails.
Yeah I bought my upper body armour for previous shoulder and chest snowboarding injuries. Touch wood but since the armour I've only had one upper body injury (chest) and it was relatively minor.
For the park I 100% agree on as much armour as you can comfortably wear. For regular all-mountain riding I'd say two pairs of lower armour is overkill. Get a single piece of gear that protects the injured area as best as possible, and then spend the rest of the time learning to fall better.
I perfected the turtle roll last year. If I'm wiping out on my back I go into the fetal position and land on the spine protector, then slide down the hill on my back like I'm in a turtle shell until I can get the board facing the right direction to plant it and stand up again.
Some of us were lucky enough to learn in our early teens (or earlier) when falling didn't hurt so much. For riders that learn later in life, I strongly encourage learning to fall and practising it. I fall every day out on the hill and I'm proud of it!!!