@OneNewLeader Honestly, I could go on about this all day, every day.
@Hotoutisntit I was referring to when your friend sends photos of 'good times' and family days out -she's almost certainly trying to reassure both you and herself that, although she knows you know just how dangerous her partner is, he's got some redeeming features too and isn't 'all bad'. The cognitive dissonance of a cowed, coerced spouse is complex. And it's only natural, in the face of pity and knowing others feel sorry and concerned for her, that she tries to signal some kind of normal functioning in order to preserve a sense of dignity.
You're in a position to have done what many women in coercive and violent relationships dream of doing, yet find themselves, for a variety of reasons, unable to do. This almost puts you in a kind of empathy deficit, as does your feeling of needing to understand what makes it so: because you could leave your abuser, you struggle to make sense of why she feels she can't. If you really want to be a support and advocate for your friend, you may need to let go of the idea that you need to understand how she is functioning and reasoning. There are probably very few things in her life that really make sense even to her -maintaining a sense of confusion and uncertainty is standard DV playbook for forging and maintaining a strong trauma-bond. I understand that it is infuriating not to 'get' how she's making choices that put both her and her DC in harm's way. But she is. You can either chase your tail trying to understand, or accept it's outwith your capacity to make sense of, and choose to support her anyway. But please support her from a deeper place than trying to reason with her -she has probably been reasoning with herself, and judging her own choices for years.
You say she doesn't seem afraid of him. Abusers rely on family keeping up appearances and not letting the DA 'cat' out of the bag. He doesn't want her cowering and shrinking from him when others are around -he likely doesn't identify publicly as a violent abuser, so your friend knows not to act as if he is. That's my guess.
Edited to add image credit: David Mandel 'Stop Blaming Mothers and Ignoring Fathers'.