If she hadn’t already burned her bridges, then I would be volunteering myself to fix her fences if she purchased the supplies. Sounds like she has a basic fence, but needs a kick-a** charger that will probably cost her a $100+.
But, since it sounds like you have had numerous discussions with her, and she has not exactly been gracious to you when you did help her out, I would be calling Animal Control, also. I might also call the police and inform them that her horse has walked down the road numerous times. Even if it isn’t a busy road… Busy or not, a horse in the road is a disaster waiting to happen.
And I would start keeping a log every time the horse shows up. Write it on a calendar, snap a few photos with your phone, etc. Something to show the “officials” if there was ever a need.