So far, so good. I can't see anything wrong with extending one line of research in new directions. In fact, that's what the scientific method is all about. We do similar things with news stories. If one newspaper looks at an issue in their hometown, we may look at the same issue here.
It becomes plagiarism when you lift parts of someone else's work without properly crediting it. One mention of the earlier work doesn't' cover all sins.
Not only is the layout and structure of Meehan's dissertation remarkably similar to Boening's, sentences and entire paragraphs are almost identical. The data changes, but the wording of Boening's dissertation echoes through Meehan's paper.