My experience with people like this is they want the drama. They live for the drama. This means nothing you do is going to change anything. The ONLY thing you can control is your reaction. The other thing you can control is your participation.

Visualise her moving onto something better. Wish for her to find something else so amazing for her, that she chooses to move on. You will find peace in her absence from the group. I did this with regard to my boyfriend's roommate. A few months later the roommate moved out. I've been married to my husband for thirty years now. I'm sorry, but I don't see another solution, as difficult people plant themselves there to create drama.