Horses get hurt. It is a fact of life. Even in the best environments there are those who simply find something to injure themselves on or get into it with a "best buddy" and get a kick which causes an injury. We try to do our very best to prevent these situations but sometimes it is impossible.
Containment or the lack there of is a elemental issue if you take horses as boarders. This is where you need to be 100% sure. As a BO you have a legal and moral obligation to ensure that your facilities are able to safely and securely accommodate the horses that owners place in your confidence and expertise. If you knowingly placed your boarders horse in the paddock which had proven to be unsecured in the past you may have made a poor decision. In good faith you should at least be open to discussing the situation with the owner. Once you admit any degree of liability it is simply a negotiation at that point. This situation may require some degree of personal liability because you were aware a potential problem existed and did not make the necessary changes to prevent a future occurrence and thus an animal was injured.
It sounds as if this situation will be watched by your boarders to see how you handle it. It will likely be a good test of your business, communication and client relationship skills. Good luck.