After reading the other responses, I have a slightly different perspective. Some people, and based on the history of your and Nina’s relationship, are locked into a ‘rescuer’ mode. They consciously or unconsciously seek out others who are wounded, broken and struggling.

While being there for other people is a good thing, doing it as a rescuer or savior is actually codependency. The rescuer gets a sense of personal value from being the savior and the person playing the ‘victim’ role doesn’t have to stand up and face their stuff because someone is willing to do it for them.

A lot like the dynamic the two of you shared, over and over, including the moment when she gave you the ultimatum to tell about the infidelity or she would. Then, you chose to let her handle your marriage issues instead of owning your stuff.

When Nina saw how devastated your ex-husband was, she found herself a new ‘project’, a different person to ‘save’. These types of behaviors are learned in childhood, if Nina had a parent with an addiction or was the one who always had to smooth over the family wounds. She’s likely thinking this is healthy behavior, when it’s not.

For you, focusing on your own healing from the codependent pattern would be the key, as Nina and your ex will either be able to move past this and have a good relationship or they will play out codependency with each other. The pattern keeps showing up until one person gains awareness and heals their original childhood wound.

I highly recommend the book, Codependent No More — speaking from experience as a former ‘rescuer’.