How can you say it doesn't help? It may not help in every case, but a weapon, of any kind is a tool. It depends on the person who holds the tool to determine whether it's helpful or not.
Quick question sportcheck................where's she gonna stash the weapon so that her abuser doesn't find it if she's STILL LIVING WITH HIM??!?!?!?!?!?!?!?!?!
And................even if she does find a place to store the weapon, is it going to be readily available to her the next time she starts to be abused? Got news for you, as a person who lived with an abusive stepfather for 3 years from age 5 to 8, the abuser is going to know EXACTLY what the abusee is doing 24/7, because control is part of their game.
Might wanna try turning on your brain sometime.