I just turned 33, but when I was 16, I started cutting myself. I didn't tell anyone and I wore long sleeve shirts so nobody would know. I didn't know how to express my emotions, so I would bottle everything up inside until I couldn't contain it anymore and my way of 'emptying' the bottle was to cut myself.
I started off doing it only occasionally, but it wasn't that long before my 'bottle' was filling up so quickly that I was doing it every couple days. It was never about getting attention, it was about trying to cope.
Someone earlier said that if someone really wants to kill themselves, they will do so successfully and I have to agree with them. I tried to kill myself once, but I was only half serious about it. While I did swallow enough pills to kill myself, I did it when the chances of someone finding me were high. If I really wanted to die, I would have done it when I had no chance of someone finding me. (I ended up in hospital being force fed a disgusting thick black liquid made mostly of charcoal)
That was really the first time anyone knew that there was something wrong with me. I think it was my way of crying for help and it could be your friend's way also.
Speaking from experience, there are only two people who can 'fix' her and you aren't one of them. She needs to either sort this all out in her head herself, or she needs to seek psychiatric help. The most you can do is try and guide her to the help she needs.