I was recently asked "What do you want from me?"
I'm curious about the context of this question -- what was going on beforehand (i.e., a related discussion, something low-key, something intense, etc)?
I think NotAHero's suggestion that she may already "have an answer in her head" could be close. That is...
I wonder, a lot, if pwBPD use words/talking/conversations not to communicate/share information, but rather, to express feelings. We can get caught up in the meanings, in the words that were said, and (understandably so) treat the words they say as meaning what those words mean. Instead, I'm starting to suspect that pwBPD use words to express emotions, kind of "coming at it from the side", not directly/with the dictionary meaning of the words. In fact, despite many appearances of competence, togetherness, emotional sophistication, using "sophisticated" vocabulary, etc, pwBPD are rather stunted in emotional expression.
So, yeah, the vibes I'm getting at the moment are -- she is feeling anxious and insecure inside, and hopes/believes that if she can just hear "the right thing" from outside herself, from you, that she'll feel better inside.
But, because those feelings are coming from inside of her, there's nothing anyone on the outside can say that will help her insides feel better. Even if you came up with the most perfect words, she might still feel anxious, afraid, rageful, etc, because the words you said didn't change how she felt, and that's what she wanted.
All that being said, context could help us brainstorm some low-intensity responses or some moves to take the temperature down, because I suspect it's less about coming up with the right answer, and more about taking those moments and making them the "least worst" possible -- finding a way to balance her need to do some emotional expression with your need to not be her emotional punching bag.
Any of that ring true?