I've been feeling a bit uncomfortable with some of the comments about Louisa being stupid to go back to Jack, that she knows what she's like, that she is an intelligent woman with a high powered job so it's her own fault.
I realise that this is because I have been in a similar situation (though my narc was way more covert and I wasted 6 years trying to help him) so it is triggering to me. It may not be what is going on in this situation at all, but it does look very similar.
I hope this link works, as it's the best information I have found on the subject to explain it to people.
Trauma Bonding