Girl I totally get you why you want to warn her!! I did the same thing when I was with an abusive ex, I reached out and warned the girls he was haven intentions for ..I even done it while dating him (yeah he cheated a lot...long distance relationship suck when your partner is a douche) but you know what them girls I thought I was helping to warn...didn’t actually care they acted all nice and grateful but at the same time questioned him who I was to which he then told them I was a psycho and they believed him as all I was to them was some random girl, I gave up trying to help them in the end and thought if they are dumb enough to go with him even after a warning then they can’t say they’ve been warned! You’re better off just forgetting about him and them in the end and think of your own happiness now you’ve escaped narcissist! Good luck! Btw you’re more than welcome to message me if you ever want someone to talk too about this.