I found out (after the fact) that my older son did this for a time. He has scars on his upper arms from it. He went through a VERY tough adolescence.
He outgrew the cutting (though he still likes his tattoos--the piercings he's allowed to close up). I'm not sure exactly at what stage the cutting happened (he was living with his dad), but I believe it was after he had stopped the therapy that followed after his year that he spent in a youth home, but before he got past the depression that dogged him as a teenager. He seems emotionally healthy now, though I think he still has a tendency toward depression. He's mostly happy with his job and his relationship. If either of them took a dive I have a feeling he'd struggle for a bit before righting himself again. Thankfully he is pretty negative toward alcohol or drug abuse--he's strictly a social drinker/occasional toker, to my knowledge.
Hugs, hopefully this will be a passing phase, though it's good to pay attention to it and for her to get some help.