BPDw looks at me and says: "Are you going to address that?"
That sounds like, "Are you going to rescue/protect/save me?"
This is a moment for her to be accountable, and she bounces it to you.
"I will hear you out, and I will hear S12 out. I have a hard time thinking straight when two people I love have strong emotions, so I might need to collect my thoughts on this. Let's take a time out now and regroup to go over this when things cool down."
Or something like that? She wants to pull you into the drama triangle with her as victim, S12 as perpetrator, and you as rescuer. If this is a chronic dynamic, she will be angry that you aren't rescuing -- it takes skill and practice to move out of that role.
Rescuing just teaches S12 he is at the mercy of triangulation dynamics, the source of a lot of conflict. Instead, you have to be a coach from the sidelines and not get dragged into a rescuer/fixer role.
What incident or controlling behavior was S12 responding to?