Pedestrian bridges are generally only for use by pedestrians, and there are usually signs posted to that effect. The skateboarder should not have been on the bridge at all. Even if the skateboarder yelled out a warning, his wrongful action in riding his skateboard on the bridge was the cause of the accident. Furthermore, he should have been able to notice that your aunt was somewhat elderly and that a shouted warning may not have been sufficient. The skateboarder is most likely going to be liable for your aunt’s injuries.
The difficulty here is that the skateboarder is unlikely to have a great deal of money or assets to make it possible for him to pay a significant award of damages. If the skateboarder was a minor, your aunt may also be able to sue his parents for not adequately supervising their child. And in some cases, depending on the jurisdiction and how the policy is written, homeowners’ insurance may be available to pay any damages award.