As a supervisor, I would not ever, initially bring in another peer. If in fact there is a need for disciplinary action, asking a peer to be involved would be poor practice and would likely make the floor nurse feel diminished. If there is another RN who is directly involved in an incident, of course include them.
There are many ways to handle this. If you are a manager and your staff respects and trusts you, there should be little problem having a dialogue with a staff nurse.
When the situation calls for a third party, you can ask another floor manager, an HR person, a liaison…lots of people who can be an objective third party.