transformie--sorry about this. I had a looney tune like this who turned on me so fast it made my head spin (all because I finally answered the phone after she had called my work phone and cell phone about 20 times in 10 minutes and told her I was working and would she please stop calling because I needed to get work done).
She sounds like a twin to the person you are talking about. Sent me a long email making herself out to be the best friend on the planet and it is her fault because she expects people to treat her the same way she treats them. I always wondered if she figured out the reason she has notfriends is because no one wants to be around her. I cut her slack because I knew she is an ACoA and many of her behaviors were due to that--but she chose not to seek treatment.
I was able to just walk away. Did not even feel guilty about it. You are in a different situation since your are tied together in the business. Anyway you can find someone else to replace her?