I think "whom" because "we" is the subject and "John" is the object (the one being sent).
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AnonymousWhich one is appropriate here?John Smith, a key person in our accounting department, whom we sent to....
“, a key person in our accounting department who/m we sent to assist… ”
AnonymousWhich one is appropriate here?“, a key person in our accounting department, whom we sent to assist…" (Please note that a comma after 'department' should not be left out.
“, a key person in our accounting department who/m we sent to assist… ”