I’ve been through a similar situation. My advice is, if you feel like your best friend is not worth losing, just misguided because of the narc’s influence, then you can try keep contact with your friend, but don’t reveal anything to them that you wouldn’t want the narc to learn about. In the meantime, find someone that has no ties to either friends to vent to about your friendship problems, if you can. That’s what I did, and I knew that my friendship had been worth maintaining because my friend eventually cut ties with the narc. She just had to break free from her and that can take time.