Metta is a practice and emotion, rather than a concept.

In regards to the doormat notion, it reminds me of one of my teachers. He said that often children will throw a tantrum when they don’t get what they want. However, if they want a sugar sandwich (gratifying but not nourishing) then even if they throw a tantrum, we don’t give it to them. If we do, we are perpetuating their suffering.

So, it might actually be the kindest thing to do to simply tell her that if she talks like that you will hang up or leave, and follow through consistently. It might help her learn that there are consequences to her actions, which is a huge step in recovery. The tantrum will follow, but without your side there to sustain it, it will blow itself out.