If she is plotting revenge then she is not that unstable or feeling as abandoned as she claims.
Some people thrive on nothing but negativity. That is how they live their lives and they do not know any other way. Her FOO maybe be evil but it does not sound like she fell far from the tree herself.
Many people that are raised in a negative environment have a hard time ever looking at their own actions. Any and all reactions are blamed on others and they have a tendency to lie about the real story. They omit their part, change their part or flat out lie about their part because they want you on their side of the playing field. Even if they are caught in the lie they can and will spin it so they can justify their actions. I only did this because she did that…It never ends.
This person does not sound accountable for her own life and actions, she is blaming every one else for everything and plotting revenge because of it with her own rationalization thrown on top for good measure.
The only way to deal with that type of game is to simply step off the field and refuse to play.