I say this as a therapist and as someone with a friend somewhat like this. Not as bad, but with other mental health issues that I cannot deal with anymore.
You are right that depression is intertwined with negative thinking and huge cognitive distortions. But, Irulan said it well. You are enabling her to continue this behavior when you don't set boundaries around what is acceptable. Be clear, use a neutral tone, and then detach. I know some people feel this is "mean," but you need to think about you. Therapy seems to have worked for you, and I wouldn't surround myself with anyone with these traits. I have detached myself from a close relative because of something like this. I just can't take the negativity, and refusal to see the glass half full.