Her multiple choice was poorly designed (10 possible answers). A large amount of time would be spent reading through these answers until getting to the correct one (imagine if it was the last in the list). If she did not want to reward guessing, then she should not have multiple choice - only short answer (granted it is a bear to grade but she is holding her students to a high standard).
Also you can define any test in such a way that the majority of good (perhaps not great) students would fail it. I had Physics tests in college with a 22 average, and that average reflected the poor quality of instruction and test design as opposed to the commitment of the students.
On the flip side, the fact that these are non-science majors should not have a bearing on the test and performance expectations. Engineers and scientists take the same English/Social Studies/Humanities courses as those majors, and there is no lesser expectation on them. In fact when I went through the highest grades came from engineers in these classes (I was an Engineering Freshman taking a Junior level core Communications course, and I had the third highest grade in the class). Virtually all of the highest grades came from engineers.
If you cannot pass a real science or engineering course in college, then you have no business being in college. The same can be said for English, social studies, communications etc. No ghettos with inflated grades - everyone takes the same science (Intro Chemistry, non-Calculus based Physics, and Intro Biology) and at least Precalculus.
Easy way to discourage guessing is penalty for wrogn answers. My professor in first compuer graphics course used this as a part of final. 5 answers you get a right one you get 20 pts you pick a wrong one you get -15, if you pick nothing you get 0 adjustment