Can only a portion of a longer document like a thesis, proposal or dissertation be reviewed or edited?
Of course.
But know this: Consistency across a document is only achievable when a complete document is edited!
A complete edit is what could make the difference between an acceptable paper (or not) and an excellently presented paper.
Why you might wonder? Simple.
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The editor is taking note that the same formatting and the same language and preferences are being used from chapter to chapter.
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Certain crosschecks can only happen if the whole document is available:
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Between headings and TOC, and figures/tables and their lists.
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Between the in-text citations and the references.
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But of course, the choice is always the student’s 
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One can certainly start with having individual chapters edited. But at some point, an edit that covers the whole document should be considered.