Every Employee Handbook should include a defined grievance procedure, which outlines the steps an employee must take when he/she has a problem, from potential violence in the workplace, to harassment, or even a common complaint.
The very first step should be to encourage the employee to talk to the person they feel is causing the problem. 90% of problems go away after this, without the necessity of management involvement.
The next steps should be hierarchical – if you’re uncomfortable going to that person, or unhappy with the outcome, go to your supervisor, and so on.