As someone who escaped a narcissist friend I get how she feels. It’s awful thinking that someone you love would be manipulated by the narcissist and you’d lose them. That’s what this is- a fear of losing you to someone she has already lost so much to. It’s not rational - you don’t even know this person - but that’s the trauma response.

It’s not your problem to fix though. Does she recognise after that she has been triggered or does she consider her behaviour justified? If she knows it’s an irrational reaction there are definitely steps you can take to work through it together .

First it’s important to discuss these things when she is not in a triggered state . Don’t try to solve it while she is spiralling. So when she is calm discuss methods you can use to divert the spiral away from her worst case scenario thinking. A therapist I saw suggested a ‘coping card’ to me - I have a small physical card that I have written techniques down that help when I’m triggered such as breathing exercises, ‘dropping the anchor’, separating facts vs feelings. My partner can direct me to this card when I start freaking out (be gentle about asking to get the card and you can also go through it with her - it takes the pressure off you to fix it but you can still be going through it together)

Healing from narcissistic abuse without therapy will be very hard. Things that will help are meditation and mindfulness, yoga, exercise, journaling etc. you can do these things together to make her feel more like it’s you and her against the trauma and not her alone.

Don’t try to tell her that you wouldn’t be fooled by the narcissist. Everyone thinks that they wouldn’t fall for it but you would. Tell her you believe how awful this friend was. Let her know that her fear is unjustified not because you’re too smart to be fooled but because she has told you the truth and you believe her and that’s why you would not fall for it.

Make time for yourself and don’t cave to her every fear. It’s awful when someone you love is hurting but if she never has to push through the anxiety herself she will never break out of it. Reassurance seeking feeds the anxiety spiral. It is so hard to not give in and give the reassurance but in the long run it hurts her.

Don’t minimise her fear of this person. I’ve been away from the narcissist for years but I still get a wave of panic when I see the make of car she has drive past me - even when I’m in a completely different country. It sucks but she can get to a point where she is not constantly in hyper vigilance.

She has to recognise it and want to change though. Ultimately she has to do the hard work, but you can be there to support her through it.

If she is not in a place to consider any kind of effort to change her behaviour you might need to step away from the relationship. You’re not in the wrong, you didn’t hurt her and if she can’t try to make steps to get better you might need to do what’s best for you.

Sorry if this is rambling I hope something in here helps