The Illegal Immigration Reform and Immigrant Responsibility Act of 1996 clarified the rules and regulations for deporting non-United States citizens. Any person who is not a United States citizen can be lawfully deported at any time, and even temporary citizens who possess green cards and visas are susceptible to deportation if they commit criminal offenses, such as prostitution, fraud or homicide. Anyone can report a non-United States citizen for investigation, but only the proper government agency -- namely the United States Immigration and Customs Enforcement -- can enforce deportation.
Have a valid reason the person should be deported. Reasons that could lead to deportation are lack of a legal permit to be in the country and hold employment, or entering into a fraudulent marriage for the sole purpose of obtaining citizenship. Gather any evidence supporting these claims, and be ready to provide the information to authorities upon request.
Contact the United States Immigration and Customs Enforcement (ICE), a division of the U.S. Department of Homeland Security, by calling 866-347-2423 or by visiting its website. Explain your suspicions to the agent who answers. Offer the person's name, address or any other information you can provide to help the department investigate your claim.
Follow instructions, and offer to serve as a witness if the case need further investigation or goes to trial. If detained, the person in question could be held for months until either a trial or deportation occurs. Request to be notified if a trial is held.