Thanks a very good question PANTS, you sound like you care for her very much. Id say if you're going to take her you should consider a few things.
1) What kind of person she is. Personally I don't see anything wrong with a 14 year old rocking out at home to music she can't understand the lyrics to, however at a concert understanding the literal meaning of the lyrics or no there is that incredible vibe that goes through the audience that *is* sexual, sometimes violent and at times a bit hysterical.
2) Keep in mind you're taking her to see a band who's lead singer fish hooks his face. Here are some pretty good pictures taken during a concert that should give you a feel of what the average diru concert is like :
musicpix.net
That level of intensity on stage isn't for everyone and seeing someone fish hook themselves or cut themselves with their fingernails or razor blades isn't something you'd like to show her you should let her parents know ahead of time so they can think on it.
3) And I know some people say this isn't a problem but I've taken younger members of my family to a concert before so in my experience its something to worry about :/. And that's how *physically* little she is, where you'll be sitting and how many concerts she's been to before hand. If you're sitting in an area where there's bound to be a lot of energy and she's very small I suggest working something out so you can always be near her. Diru gives some of the most intensity concerts I've every been to and the first one by them I attended at the Wiltern had people in the pit literally passing out and having to climb over the moshers to be helped out by security so they could see the on site medic. lol I still remember the amount of girls in wheel chairs or laying on the ground just trying to breath deeply and shakily holding the water they gave them to sip over in the sick/rest area of the hall after the show. I was almost one of them. So if you DO go ahead and take her make sure you work something out where you can be with her at all times, perhaps wrap your arms around her to keep her from getting mashed (usually I stand behind my cousin at a show and when the crowd moves I put my arms in a loose circle around her to keep her with me and make sure some dumb butt doesn't try to mash her).
I hope whatever goes on you have a good and safe time!

And yes a lot of the crowd is female but the concerts I've been to are equally mixed. I think JREF tends to be a little more female on the diru threads. lol