One would assume that access is given to Customs & Dept of Homeland Security too.Maybe, but even so, from giving access to the department, to allowing border officers to prove to foreign citizens that they can read their PMs goes a long way.
Using Facebook's private messaging, with or without NSA spying, is very stupid because we know Facebook stores all of this forever, including after you "delete" your account. That could make it accessible even in a non-criminal civil dispute.
She lives in Germany, which has a widely different fact gathering process, being much more restrictive and protective of the defendant's privacy.
Moreover, she's 18 and from a country which is probably much less lawsuit-happy than the US. It doesn't seem stupid to me at all that she didn't consider being sued when sending PMs to her host.