The short answer is yes, you can legally discipline the supervisor now. My first question would be why did it take so long to learn about this? Did an employee who claims to have been adversely affected by the relationship recently complain? Or, was this something that you learned about through workplace gossip. I think your approach should differ depending on the answer. If you do choose to address this now, you should make sure to do a thorough and complete investigation. Sexual harassment often comes down to a he-said/she-said situation. It may turn on credibiity determinations. I think it would be very difficult to find the supervisor guilty if the subordinate refuses to cooperate with the investigatiom. You would be well-advised to consult with an experienced employment attorney.
This response is for informational purposes only and does not create an attorney-client relationship.