Amateur Diagnosis - An Amateur Diagnosis is when a non-qualified individual confronts someone whom they suspect suffers from a personality-disorder and shares this belief with them, usually in the hope that this revelation will help to improve the relationship or the situation.
It's common when individuals who care about someone who suffers from a personality disorder first learn about personality disorders and begin to understand the strange and frightening behaviors in a better way to experience a great deal of relief known. This is sometimes called the "Lightbulb Moment".
This is sometimes followed by a desire to share this new-found knowledge with the person who suffers with the personality disorder. The Non-PD may hope that the personality-disordered individual will see that the diagnosis is accurate and start working on their recovery.
This could be well received if the other party has approached you to ask for help or is already concerned about the consequences of the choices they are making. Then they may have opened a window where it can be discussed in an open way even if only temporarily in a moment of clarity.
However, it is often the case that a personality-disordered-individual becomes defensive when someone else suggests that they may have a personality disorder or other mental illness.
It's also common for them to turn it around and say that it is the Non-PD that has the problem or to create some diversion or list of excuses for their behavior. This typically leads to circular conversations which can lead to further conflict.
Another common result is for the personality disordered individual to hoover the Non-PD or agree to get help. This defuses the immediate conflict, but the follow-up may be short-lived. This often leads to an Abusive Cycle.
Despite these challenges, the urge to say something can be overwhelming and it's still common for people to try this - often in the heat of a conflict or as a last-ditch effort when they feel they have nothing to lose.