FWIW, I do believe that our priest brought up in inquirers class that idea that confession was not guaranteed to be 100% confidential and some situations, similar to the one at hand that would result in something not being confidential.
Second, I can say as someone in the helping professions that the laws of our state spell out that some things are not able to be kept confidential and that some people are mandated reporters. (Don't remember if priests are on that list or not.) We notify clients of this up front and have policies in place spelling out how to proceed, and still having been in the situation of actually making a report on a few occasions, I can tell you its not easy, fun or casual. Sometimes, however, it has to be done for the protection of others.