As others have said, if Verizon says the phone will work there, it should. Cost is another issue.
There is an extensive thread on the subject of German SIM cards here:
http://www.flyertalk.com/forum/germa...d-germany.html
That said, there's not a lot in that thread regarding burners, but they sell them all over the place. If she wants something easy, she can stop by the o2 store at FRA and pick up a
Samsung Galaxy Y for €50, which includes €5 of credit upon activation. The reps there should be able to get her set up on a data plan (either 1GB for €15 or 5GB for €25).