The legal term is called "privilege" or privileged communication. This is granted only under certain conditions when the sanctity of the communication is necessary for the function of society. It is dealt with under the rules of Evidence. The Federal Rule is number 501 and states have their own rules but it is doubtful that you would have such a privilege, especially since the act you witnessed was not a "communication" that would be private. Some well known privileges are, per 501:
- The Executive Privileges.
- The Attorney-Client Privilege.
- The Privilege for Marital Communications.
- The Physician-Patient Privilege.
- The Priest-Penitent Privilege.
The function of society would be disrupted if these people were called to testify what others told them in confidence. How could a person on trial for a crime tell his lawyer the full story for his defense if his lawyer could be called to the stand to testify what his client told him in confidence? No trust would exist. It would seem doubtful that you have such a privilege given the facts you list.