Perhaps she posted her intent to work illegally in a public facebook post and not only via PM? Or perhaps her host family was under the impression she was official and triggered some sort of alert when they were arranging things (if she for example, listed them as a contact when she got to customs). The typical au pair process from Germany is quite strict and time consuming to get a visa for a 1 year stay, I've had a few friends come to the US to do it. You most definitely need a work visa to be an au pair and she was attempting to violate her tourist visa in two ways, by overstaying the 90 days allowed, and by working. A lot of things could be going on here.