...whose been with each other for a long time - if there was a lot of drama, they are more likely to stay in each other's business. They might even opt to stay with each other out of familiarity.
Why is this? Because they've put up with each other's crap for so long and are completely comfortable showing their worst side to each other. At this point, trying to start a new relationship means they have to change and show up for the other person. Also, they will have to adjust to the new person's flaws, which they don't have the energy to do. Therefore, might as well stay with the devil you already know.
Honestly, the only way this cycle ends is if the woman has self-respect and cuts off the man. Otherwise, he'll always use her as a supply for when things aren't going right in his life.
In fact, it's women who don't have self-respect that create the environment for these toxic on and off situationships to exist. Until she has a new man who is serious about her and marries her, she will keep this cycle going with the narcissist.