Did the 19 pounds include hay? When you say feed do you just mean supplimental?
The reason some people board is because they can't be there to care for their horse themselves. If she was paying for the horse, she did not abandon it.
Did you have her permission to ride, train, and shoe this horse?
If I had to be away for an extended period of time and someone had been riding or Anything without my permission, I would be quite upset.
Even if she is just a fair weather horse owner who just didn't feel like going to see her horse for a few months, as long as she is paying her board she has done nothing wrong.
If you were messing with this horse without permission, that is your bad.
If you did have permission, then it was your choice to spend extra money on it. You were lucky to have a free horse to ride for a while.