It's an emotional thing, I used to do that. It's almost a way to get anger out, plus it's sort of like cutting. Some people cut because they feel like they've been punished so afterward they can allow themselves to be happy with their lives. The leg breaking thing is the same
If she actually breaks a leg she will be amazed at how much it hurts and how long it takes to heal. You have to do exercises when the cast comes off. I nearly fainted when I broke my wrist. Good thing there were people to get me to the emergency room.