Why can I see through him and others can't?

Okay, just humor me for a second...

So, a family member of mine ran into my ex-N at the grocery store with a woman this past weekend. Now, this woman wasn't the new OW he's supposedly in love with and moving on with but an old OW who has been lingering around for years, even before I came into the picture. This woman is almost 20 years older than me and 10 years older than him. She is absolutely in love with him and allows him around her child (who is in his early adolescent years). They do some business together and they have also had a personal relationship too, which is casual and ongoing. In other words, she's a part of his harem and under his spell.

Now I've been to hell and back from dealing with this idiot. I've seen the good, bad and the ugly but primarily the bad and the ugly. I'm careful not to have him around the son I share with him because he's crazy, unstable and a bad influence but she has her son around him all the time. My question: Considering she's been in the picture much longer than I have, how can she still run up after him like a love sick puppy and not see him for the monster he really is? Do Narcissists expose themselves to some people and not to others?

Generally speaking, I've noticed this with a lot of people who have seen him mistreat me but who are still good friends with him and have, instead, turned against me, even though when they see me, they talk about him like a dog (obviously duplicitous people). Can anyone explain this? What's wrong with these people?