Let's hold on before we start inferring things about the OP and everything that goes with referencing Germany on the internet....
Title clearly states "Hypothetical What If Scenario"...obviously wanted to know what most people would have done, didn't say he reported it or did anything besides talk to her. Frankly, I think it's a very thought provoking question. I had never heard anything about any TCA that allowed minors to conceal or carry on anything other than private property (hat tip to JayC), so I've learned something. I would assume you'd see a minor being given permission to carry in a setting more like a family owned store (maybe a LGS) more than a traditional place of employment, which statistically is more likely of a place for the OP to work. As far as reporting her, for me personally I think it would depend on the person. I don't know if there is a legal obligation to report this type of potentially illegal activity (I would assume not), but assuming there isn't if the person is squared away i would probably let it slide with a warning that they should be more careful in regards to who knows about it and how they are about hiding it. We're all gun folks here and you can see the division. Imagine if the person who found out was ignorant about guns and the laws associated with them. I imagine it would cause quite a ruckus.
Just my $.02.