"Fracture" actually means broken. "Hairline fracture" would be partially broken. It looks like the break is in the fibula, down near the ankle?
That's not a terribly bad way to break your leg, from what I've heard, and it doesn't look like break is too wide. Still, I'm sure it hurts a lot. Did they delay putting on the cast because her leg was too swollen? That happens sometimes. Does she have a follow-up appointment?
Best thing is just for her to rest with her leg propped up, way up above her head as much as possible. If they gave her pain medication, she should take it, otherwise Motrin is particularly good for bone pain. Also, she can put an ice pack behind the knee to ease pain and swelling.
Until she gets in the cast, she shouldn't move around much, just rest with the foot propped up. When the pain and swelling subside, then she can maybe be more active with crutches/walking cast.