Here is a link to the Maryland rules of the road:
http://www.mva.maryland.gov/Resources/DL-002B.pdf
I was curious because the article didnt indicate what she was doing was actually illegal. I can't find a law forbidding the practice of driving the speed limit in the left lane in the MD ROR.
Unless I'm missing something, this looks like an officer's judgement call. I agree about staying out of the left lane but I think she can contest this successfully.