Exchanging co-authorship for proofreading?
I think this is professionally unethical. Other fields might have different views, but per the International Committee of Medical Journal Editors, authorship is based on


  
  Substantial contributions to the conception or design of the work; or the acquisition, analysis, or interpretation of data for the work; AND
  Drafting the work or revising it critically for important intellectual content; AND
  Final approval of the version to be published; AND
  Agreement to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.
  


Since this graduate student has no intellectual stake in the work, this is basically lying. The fact that the student would benefit from a publication (and you, if you are their adviser) makes the conflict of interest all the more apparent.

The appropriate place for such a contribution is in the acknowledgements or not at all.