Glad your boy is doing so well.
From the OP I get the impression that what is sticking in your craw is the fact that you used to be friends and that you helped her out when she was just starting out.
I totally understand your desire to get back at her for the way she treated your horse and what you perceive to be a lack of gratitude.
Take the high road on this one and dont look back.
You can write her and express your dissatisfaction about her lack of care and let it go at that.
Getting the legal system involved will just create more tension and you probably wont win very much when you consider the time and emotional effort you will have to invest into a legal case.
It is very difficult to prove to a legal standard that the BO is responsible for your horse’s lameness.
For your own sake you should pay what you owe and be done with it.
You are most likely never going get what you are really looking for, which is an admission of responsibility from your old BO and an apology.
Hope this helps