If you hear a coworker telling a visitor wrong information how do you handle it?

If the coworker is your boss or somehow superior to you in rank, then do not correct them in front of the visitor. I myself have gotten in trouble for doing that myself and have learned my lesson. If it is a coworker that has the same authority that you do, then unassertively suggest the correct answer. For example say, "Joe, I believe that it's...." or "Are you sure? I thought you had to...." If the coworker is still adamant in his or her information, don't push it because it would be unprofessional to bicker and fight in front of a customer/visitor. Either wait for that person to walk away and discuss it and maybe you can chase them down if its that important, or discuss it with your superior if you think the coworker is being stubborn about issues like this.