Can a Person Sue a Company for Defamation of Character & Slander?
If someone believes your company is spreading harmful lies about him, he can definitely sue. Hundreds of people have done that to newspaper or TV companies they believe defamed them with lies. If you defame someone on the job, he can also sue you individually. With the emphasis on free-speech rights in American law, it's usually difficult for the plaintiff to win, however.
Slander and defamation overlap. Slander is defamation of character spoken aloud, where libel is defamation put into print -- including online. What you say is defamatory if it's false, harmful and you say it to a third party. If you only say it to the person you're lying about, there's no damage done. The law considers some false statements automatically harmful, such as claims that someone sleeps around or attacks on their "professional character."
Truth is an absolute defense in defamation cases. If what you said was factual and accurate, you're on safe ground. If what you say is clearly meant as opinion, not a statement of fact, you also have a good defense. In advertising, you have the defense known as "puffery." If, say, you advertise having "the best BBQ in Arizona," other restaurants may feel insulted, but they haven't been defamed. Falsely claiming the competition uses horse meat in their barbecue, however, would be actionable.
Defamation doesn't apply to what are known as "privileged" statements. If, for instance, you say in a court hearing that someone stole from your company, for instance, you cannot be sued for slander, even if you're wrong. Employee references are also covered by privilege. When talking about a former employee to her potential new employers, it's not defamation to criticize her performance. However, you cannot make a false statement of fact or say things about her out of malice.
Even if someone takes your company to court for defamation, the odds are usually in your favor. The burden of proof is on the plaintiff: if you say you told the truth, he has to prove otherwise. Even if you spread falsehoods, he may not be able to prove that in court. Theoretically someone could sue even if you have a solid defense, but most lawyers won't accept an obviously hopeless defamation case.