As a parent of a teen myself this is a topic I'm not thrilled to think about, but it's SO important to make sure your son/daughter is well-equipped with the skills to recognize dating violence and END IT. Remember-this isn't a one-sided issue; girls can be physically, emotionally, and sexually abusive to boys as well.
Talk with your kids--Watch the above video from the CDC-Break the Silence; this is a great video from the CDC on how to prepare your kids.
Have you talked to your teen about this? What did you say? How did you bring it up?