Cutting behavior is often exhibited by sufferers of sexual abuse, but not always. The behavior is often a way of releasing anxiety. In my experience, cutting was used to alleviate panic attacks. It's best to research psychological information on the subject before approaching the friend so that you have a good understanding of the issue. Also, although it may seem VERY strange to the outside world, and difficult to understand why someone would mutilate their boding, it makes sense to the person that is cutting. So when approaching the person, do not make them feel as though they are weird or crazy because of this behavior. If your friend is usually open to share their feelings, you may simply ask them about their behavior making sure that you reassure them that you're inquiring because you are worried and want to better understand how they are feeling. You can offer yourself as someone to talk about their feelings and when the time is right you may suggest that they seek professional help to assist them with stopping mutilating their body. Let them know that behavior therapy helps them learn to cope with their stress and anxiety that may be causing the cutting behavior.
_________________- This also depends on the type of cutting. I don't disagree with what was said above at all, I would just like to add that it is a way to control pain (physical) when they may feel they cannot control anything else they are feeling emotionally. Also, there may be cultural reasons, or even adopted subcultural reasons as to why they are cutting. If they are cutting lines on their wrists then by all means speak to them about it, if however it is geometric patterns that leave a deep red/pink scar and not on their wrists then you need to be prepared for them to say that it is art. BMEZine has a lot of information pertaining to cutting (scarrification) and would be a great read for those who are concerned about friends and/or family members.