Retaliation is illegal. Since retaliation is another form of harassment, the victim takes the very same steps to report the behavior.
Approach the person directly and explain that his or her behavior is unacceptable. Often in retaliation situations, direct conversation is not easy and, depending on the situation, may not be safe. Writing a letter or email is perfectly acceptable. Make sure to keep a copy of the letter.
Record any and all details about retaliatory incidents including:
Report the incident, or incidents, to a supervisor or to the company's human resources department immediately. Keep a record of any report you give. The company should already be taking measures to investigate and resolve the harassment incident, but now they must address retaliation as well. This should be handled as a very high priority to keep the harassment victim safe.
As a co-worker of a harassment victim, it is your responsibility to treat him/her with respect and not partake in any retaliatory behavior. The company performs an investigation so allow the issue to remain confidential and trust that it will be resolved. Involvement in any negative act toward the victim can bring additional legal action against the company and anyone involved.
As a supervisor, if a victim comes to you, it is your responsibility to keep him/her safe from retaliation. Report any negative behavior toward the victim that you see or hear. Involve the company's human resources department to help protect the victim.