I've also had the sad experience of a relationship with someone who checks off a lot of boxes for malignant narcissism. I had to learn a lot about boundaries in order to understand myself better so as to not repeat that mistake again. There were lots of warning signs to get out of the relationship. I would have been spared much suffering if I had paid attention to them.

The main thing I look out for are boundary violations.

My ex was also a pathological liar, although not all narcissists are by any means. The closest I've read to describing his lying is 'pseudologia fantastica' or 'mythomania'. But, again, it is a separate condition from narcissism.

I didn't get the impression that golden eve's ex is a mythomaniac. I get the main impression he was crossing multiple boundaries at the same time in his conversation with her fiancee at the time. He did this by sharing unwanted 'information'. I put 'information' in quotes because it isn't even clear to me that his story that he lied was actually the truth. Why should it be?

The second thing that jumps out at me is the betrayal.

It takes a special kind of mind to cross so many boundaries at once.