What to do when you think a student is incapable of using certain vocabulary?
Thesauri are a thing.  The internet is a thing.  As such, everyone is capable of using all vocabulary and dialects at all times. They're also capable of changing their minds about how they express themselves at any moment.  One could write a serious mathematics paper entirely in Middle English (modulo terminology a bit, which will be hard to adapt, unless the mathematics is historically appropriate to the time) if they were so inclined. 

If the words are used correctly in the student's work, then in the absence of actually substantive evidence of plagiarism or academic dishonesty, then there's nothing to do.  At best you can comment on whether the choice of phrasing is suitable for the topic or not, and if it helps or hinders the clarity of the exposition.  That Middle English math paper is fairly unsuitable and inappropriate, and almost certainly unspeakably unclear, unless, say, the assignment was specifically concerned with writing in Middle English (in which case maybe the math is the inappropriate and unclear thing).

Furthermore, academic writing is often a dialect of its own.  In mathematics we often say things like "We will show..." even in single author papers, because that's the history and convention.  It is entirely reasonable that efforts at serious academic writing will not sound like natural conversation, and that anyone familiar with how a person sounds in only one of those will be surprised by how they sound in the other.