Why it is done
The workable thing to do (and stories, rackets)
Third partying is one person who is involved in the relationship talking to another person outside the relationship about the other person in the relationship. It is not about sharing something good, but about self-justifying and reinforcing how right one is about the other being wrong.
WHY IT IS DONE
It is based not on evil but a false belief that one can gain something from being right, that one can assuage bad feelings and perhaps get reinforced in one's beliefs.
In relationships, there are so many opportunities to mis-hear, misinterpret, filter to mean something else, to not really understand from an objective perspective and/or from a base of sufficient knowledge.
Two of the major things to learn about relationships and to know while you are in one are:
1. That making another wrong is usually inappropriate, in error, and/or damaging.
2. That one's interpretations, assumptions, beliefs, etc., may cause something not to be understood - and that one should withhold any conclusion until things are totally understood - and assume that one's perceptions are very likely to be wrong (until proven in a rational sense).
Jane would call David's brother's wife (who we'll call the sister-in-law) to complain about David. Her sister-in-law found that David was quite laid back and supportive of others, with no temper, but she wondered if he was different in intimate relationships. Jane had so much "evidence" that she presented and she felt bad about how terrible David was in the relationship and how he was or wasn't to her.
Her "evidence" consisted of interpretations where she often didn't really hear what was going, as her fear filters caused her to misperceive what David said or did. And she believed that she perceived it all correctly, but she could astoundingly turn anything into a negative.
She had an operation and was drugged out to stop the pain. David was at the hospital, seeing her the first moment they would allow him to. He brought his computer and a book to read, so he could have something to do while she was sleeping. He just wanted to be with her and to support her.
However, Jane declared that he really didn't come to see her and that he had just come there to do his work and wasn't interested in her and that he was unsupportive - and wasn't that terrible!!! As profoundly ridiculous of a conclusion that was, David could not shake her of it, as she absolutely "knew" she was right. And apparently none of her friends corrected that with her, as she held the resentment as part of her quiver of arrows for battle, all the way through the years and even into the divorce. She did not need to examine it nor show up for the relationship counseling, because she was right about this, among the many other things.
She kept on trying to enroll others in how bad David was virtually from the beginning of their marriage. And all it did was create more certainty for her and more damage for the both of them.
THE WORKABLE THING TO DO
The right thing to do was to address the problems with David and get rid of all the assumptions - and if that couldn't be accomplished without help, she should go with David to a counselor who could confront her and handle it - plus read up on communication and misperceptions in relationships, as she was simply unaware of what caused all her misinterpretations.
The right thing to do is to stop talking about this to third parties, as it is a way of holding onto a "story". Once seen, stories should never, ever be repeated. See the definition about what a "story" is, in this case, in What Is A Story And What Is Not A Story. What Jane is running a "racket" at great costs. you should become totally knowledgeable about what this is. Read Rackets.