Interesting but doesn't really offer a solution. If someone is asking this question they obviously don't have access to professional guidance and would like some answers.
I wounder if she tried to lead the mare into the arena, give a treat, ride her for just a bit, lead/or ride her out, then go back into the arean, give treat, so that the mare will associate treats with the arena riding.
I would start with groundwork. Work with her around the arena, lead her by it, around it, and if you can, lunge her by the entrance. Then lead her into the arena multiple times, put her away, and repeat every few days. Finally ask someone to lead her while you're on her back, and ride her in. Hope this works!
I love all the articles because they relate to a lot of horse and somtimes we need people to tell use what we are doing rong