You don’t. People with NPD don’t realize what they are doing. They have a disorder. The only solution is to walk away. You are probably never going to get along with this person and have a normal relationship with them ever. There is a certain point that you will reach with an NPDer and I like to call it the point of no return. The reason you cannot return is because they will need for you to take responsibility and accountability for whatever crisis happened in your relationship. If you don’t do this they will continue punishing you. If you decide to take responsibility because you value the relationship you are effectively giving them permission to to overstep your boundaries whenever they want.
People with NPD are not aware of their own behavior and they are not aware that they have done anything to hurt you. Contrary to what most neurotypicals believe, people with NPD do not wake up in the morning with the intention of hurting other peoples feelings. From their perspective you did something to shame or humiliate them and what you did was absolutely intentional. So they reacted. Everything with an NPD is a reaction to how you made them feel with something you said or did. All of these toxic behaviors (blame shifting, gaslighting, smear campaigns, triangulation, etc) are not planned. These are coping mechanisms they learned growing up in an abusive and unloving environment. These behaviors were necessary for their survival during that time.
Rather than trying to expose this person or avenge this person you need to understand that their disorder is what is in control. They rarely know they are doing anything wrong and when it occurs to them they do not understand their own motivation and will immediately justify their actions to avoid feelings of shame.
Shame is the basis of everything wrong because an abusive caregiver used shame to control them. They do not understand genuine love because they were deprived of it as an child. What they understood as a child can be boiled down to to different messages: 1. “You are unlovable because you are worthless” or 2. “I love you if you pretend you are someone you will never truly be”. In example number 2 the child is praised for something they are not and must constantly pretend and receive love they feel they are not worthy of.
If you want to expose a person with a disorder or even get revenge against them it would be pretty easy to do given the information I’ve just given you. But is that really the sort of person you are?
Just ignore them and move on with your life. Covert narcissist are powerless because all they can do is smear you and eventually that wears off and people who are good people will stay by your side. You don’t need the other fools. Eventually the covert narcissist will run out of gas. It was never about you anyway. It was always about them trying to either restore or protect their image. They see themselves as unlovable and they don’t want for anyone else to see that worthless person they believe they are. And the only way they know to do this is to project their shame on to the person whom they perceive as the one who committed the initial transgression by intentionally humiliating them. They legitimately do not see you as the victim. They actually see you as the antagonist. I created the following flowchart about 20 minutes ago.
You will never get closure from this person. Please understand that they have a disorder and if you tell them this they will never see it. They will think you are trying to play a game with them. Because that is how they see life. They think everyone is a hurt child inside an adult body just like them.
Just pray for them and hope that they get the therapy they need so they can have a happier and more fulfilled life. They didn’t ask for a disorder. They were abused as a child. And it may not seem like it to you, but they are actually doing the best they know how.