Treat any adult student who complains, and his complaint, with respect. It may have taken some courage for the student to approach you or another member of the faculty with his grievance.
Arrange a time and place to meet the student and discuss her complaint. In addition, ask the student to outline the complaint in writing and give this to you prior to this meeting. This will allow both you and your student to carefully consider the complaint.
Inform your supervisor of the complaint. Listen to any guidance he may give on dealing with the situation and on the student himself. He may be able to advise you on the student's personal situation or on any other complaints he has made.
Listen carefully to the student's complaint and do your best to sympathize with her. You may find that it is simply a misunderstanding that you can easily resolve. Or, when looking at things from her point of view, you may find she has good reason to complain.
Attempt to arrive at a resolution that leaves both you and your student satisfied. This may simply require that you give a clear explanation of your course's structure or organization's policy. Alternatively, it may mean that one or both of you must make compromises in order to reach some common ground.
Keep an accurate record of the complaint detailing names, dates, the nature of the complaint and any action you take. Should the student remain unsatisfied, present this record to your supervisor.