The student who turned it in has displayed a continual failure to grasp even rudimentary English language. The first draft in a three-draft assignment was the first 2.7 pages, exactly and unchanged, of the final paper. Part-way through the paper is what I originally thought was a misspelling, but happened to be a newly-created word in the research area findable in several recent research papers. The word is clearly here unintentionally, the ideas around the new usage aren't introduced, used, or explained.
The only conclusion left is that the paper is a (semi-) original work not created by this student.
Q: What do you do with this? The school will certainly not stand behind a claim of plagiarism backed only by "It's not the student's writing." Can idea density be used to support a different author claim? If the idea density is horribly low, should the student be warned of future risk of Alzheimer's?