I'm a psychiatric social worker and just want to point out that by your description of how this person behaves, she does not sound like a narcissist. She sounds like a classic case of Hysterical Personality Disorder.
The good news here is that most people with HPD are pretty normal in the rest of their lives except for wanting constant attention from every member of the opposite sex, and this includes trying to steal your man even if they don't want him, just to prove they can. This usually is the result of a hyper-critical father whom she can never please or be good enough for.
Narcissists have no empathy for anyone and you don't really even exist, and this applies all the time in every part of life. You are an appendage to them, and if you don't obey them, you must be destroyed. All personality disorders are very hard to deal with, since the person who has one will almost always deny it and refuse treatment, so avoidance is the best policy, but IMO, NPD is much worse than HPD.
I know this doesn't help much with your problem, but if you read a bit about HPD, you may get some ideas. You'll have to decide if your friend is worth putting up with this other woman too. If it was me, I would email my friend a short article on HPD and ask them if they recognize anyone they know in it, but I am at the age where I am DONE putting up with people I don't like, period. If you do that, you may have to face your friend choosing her over you, at least until this woman steals her man. Younger people like you usually have a lot more tolerance than I have, so you may want to just wait it out. A woman with HPD doesn't keep friends long, except for the ones who never have dates for her to steal. She uses that type of friend as her "wing woman" so she doesn't have to go out prowling for male attention alone. She will steal all men away from her friends, causing them to end it with her if they have normal self esteem.