OK, I think this is what I needed to hear. So if someone breaks this law is it a criminal matter or do they simply open themselves up to be sued?
It depends on what activity needs reporting and, often, in what state it needs reporting.
Illegal activity involving children (which was the example you gave) would be reported to either Child Protective Services or the police. Other suspicions of or knowledge of criminal activity (drugs, theft, etc) is reported to the police. Communicating defamatory statements would be (in most states) a civil not a criminal matter, so you would probably not contact the police but you could contact an attorney (or, depending on the circumstances, the employer).
It all depends.
I agree with commentator that most federal jobs will have security in place that limits who can and cannot wander in. If you work in a post office, however, then someone can come in and tell others you are [defamatory statement here] and there is little you can do to prevent that.
As a federal worker, you had a background check which satisfied your employer that you did not have a criminal
past. And, if your employer and your coworkers know you well, it would probably be hard for them to believe some strange character who comes in telling stories about you, especially when this stranger is from a different state (and even more especially if you have warned your employer in advance that stories may be told about you by your sister's weird boyfriend).
That said, you
can be defamed by certain statements communicated about you and, should the boyfriend decide to tell lies about you that damage your reputation, seek out the help of an attorney in your area. You could potentially sue the guy.
Good luck.