Get yourself educated and informed on coercive control, abusive RS and NPD - and the impact it has partners and how best to help them.
Then you will see the bigger picture - how YOU are his biggest threat - also how intuitive responses by you can be counterproductive, drive her to close down with him and give him more power to abuse her and also back fire on you.
The victim ends up v traumatised, gas-lit, confused, suspicious and afraid. They can turn that on you rather than the abuser. They are always under pressure and you need to be careful not to put them under pressure as well - so don’t be judgemental ever about him or the RS.
I would keep all communications open, focus on light positive times (she will need this release and contrast) you might need to fake your own demeanour at these times. Find some times or something to do together regularly that he can’t be part of.
Listen and watch. You could v subtly drop info about behaviours / boundaries that are not in the context of her RS - so that in time she will join the dots.
Mostly though keep reminding her that she is wonderful and strong and encourage her personal goals - because he will be doing the opposite, grinding her down and eroding her soul all day everyday.
You might be in this dynamic for decades. You need to pace yourself and be clear of your own self preservation and know where you can and cannot have influence.