My reason for saying you should talk to her about it is that she's likely to be embarrassed about the scars when she gets older. As someone said above, she is likely to grow out of it....self-harm is generally a teenage/early 20's 'passtime' (although some people do still do it in adulthood, most famously brought to media attention by princess Di).
One of the most frequent questions I get asked about body modification is how to remove self-harm scars, or whether they can be covered with a tattoo, or disguised by a 'proper' scarification piece. The problem is, at the time, cutting provides an instant relief to current emotional pain, and as such, people doing it are very unlikely to be thinking of the future implications of their actions.....but when it gets to the future, many people wish they no longer had the scars, however helpful the act of cutting itself was to them at the time.
You'll probably have a hard time convincing her that it's not a positive behaviour, as people who cut often have very skewed views about it (which is hardly surprising, given that it is often a symptom of depression, which completely ****s up your perspective). A good example of this is this column
http://www.bmezine.com/news/guest/20031030-cora.html (WARNING: link contains images some people might find disturbing) written by a friend of mine. At the time he wrote it, he was under the illusion that he was a transsexual, and was taking oestrogen. Since realising that trying to change his gender wasn't what he really wanted, he has said this: "My views on the topic have inverted themselves, and have given way to a massive confirmation of a very old theory of mine, namely that estrogen makes you batshit ****ing nuts." In my opinion most of the opinions expressed in the column are complete bollocks, but due to his mental state at the time, he really believed everything he said.
Rather than trying to stop her, I really think you should try and talk to her about it and get her to seek help for the underlying problem...or if you don't feel close enough to her yourself, get one of her closer friends to do it. As I said before, the fact that she shows off the scars makes me truly think that she is doing it at least in part as a cry for help. Most self-harmers who do it purely as a release/control mechanism are very secretive about it. DON'T tell her family though, they are likely to be very worried and see the behaviour itself as the problem, rather than a symptom of something more deeply rooted.