Citizenship has no bearing on German university entrance requirements. For almost all Bachelor programmes you need German. As an English speaker it's not that hard to learn, I met a med student last month who was accepted provisionally, conditional on acceptable TestDaF results and went from no German to sufficient to study medicine in three months (of 8 hours a day, 5 days a week study.)

Note that most US high school diplomas will not get you into a German university, you need APs or community college credits. I am guessing that it would get you into a Fachhochschule[1] (again, you would need German) which grants Bachelor degrees, but is more vocational than the Universities.

[1]"University of Applied Sciences" is an abombinable translation, which tells you nothing, but it's the official translation.

A note on US high school diplomas - that's not entirely accurate. A diploma plus an ACT score of 28 or greater, or a combined SAT math+reading comprehension of 1300 or greater, or four AP classes, or a minimum GPA of 3.0 with an academic track, all qualify you. Pretty much the same sort of thing you'd expect to get into a decent college in the US, really.

[http://www.anabin.de/Xml/xmlZeugnis.asp?ID=704&Land=124&...]

See barry-cotter for the general answer, but the specific is that she's also a European citizen, because my wife is Hungarian. It helps that we've spoken German at home her entire life - she doesn't speak it, but her passive knowledge is quite good and I think a good class will get her up to speed pretty quickly. So it's not entirely a dart on the world map, is what I'm saying.