If the narcissist stays in a longer relationship with his new supply source it is because his fueling needs are being met. He was able to successfully tear down any boundaries she may have had. She is trauma bonded and cannot see her way out of this toxic relationship. She is also suffering from the effects of gaslighting, cognitive dissonance, and abuse (mental abuse, verbal abuse, and many times physical abuse.) Her mind will try to lessen the effects of the abuse by making excuses for the narcissist.
If the relationship with the former partner did not last long it says she had boundaries the narcissist could not tear down. The former partner had healthy self esteem and this could not be eroded.
Some in long relationships with narcissists grew up in narcissistic households. And this toxic environment is one which is familiar to them. They don’t like the abuse and are not satisfied in their relationship with the narcissist. This is all they have ever known and don’t understand the dynamics at play. Until something happens to wake them up they may go along with the status quo, as they don’t want to rock the boat. They have been conditioned since childhood for this type of relationship. Once this person does wake up they will never want to go back to the abusive and toxic relationship. This person can and heal make changes. They can begin to form boundaries and build a healthier self esteem.
Narcissists stay in relationships where they are being supplied. What this says about the former partner or spouse is she was on to the narcissist and his abusive behavior. She saw through him early on and was not willing to put up with it. Her boundaries and self esteem are in place. This is something no one can take from her. She knows who she is! Anyone can be ensnared by a narcissist. Not all will stay and some may be discarded early in the relationship. The narcissist discards when he is not being supplied. It says a lot about those who get out early on. It means you did not play the narcissist game.