She should have fixed the fence when I asked. That truly is the issue imo. Again, I'm not an atty.
You do not have absolute proof the top wire of the fence caused the injury. As the friend of an equine attorney I can tell you - no proof - no basis for a case. It goes back to your contract. If you do not have one in place, an attorney is going to lay the blame at your feet for leaving a horse on premise without that piece of paper.
As Alwaysbehind said - you do not know when the fence was fixed. As a BO, I am up way early and in the barn very late doing things so as not to disturb my boarders. (and honestly so they are not in my way!)
I am sorry - you are upset and frustrated. Been there done that from both sides of the table. As previously stated - horses do amazingly stupid things. Your horse is young and otherwise healthy. Both positives for a speedy recovery.