What can we tell co-workers about a new employee’s sex change and transgender status?

Q. We recently hired an experienced salesperson. Dur­­ing her orientation, she told HR that she recently underwent a sex change procedure and that she is transgender. A few days later, another employee went to HR and explained that he had known the salesperson in a previous job before her sex change. This employee is clearly uncomfortable and asked for advice on what he can say to the new employee and others on the team about their former working relationship?