I have a friend that had the same problems, cutting and suicidal thoughts. This manifested during puberty and it came down to this; She hated herself. At least that was her explanation.
The pain from cutting and the attention she got trough it helped her, not hating herself for a while. But it’s an endless circle once you done it you soon need to do it again because she needed to feel that pain again.
It’s tricky business, especially when you’re a close friend. Simply ignoring it sometimes helps, but it can also make it more severe. But there is “a silver lining”. Often they grow tired of this behaviour, accepting that cutting isn’t going to solve anything. This may take years. When your friend is still in her teens, it is reasonable to assume this behaviour will fade away by the time her puberty is done toying with her emotions.
Sometimes a new experience, a little thing can trigger her emotions positively. Most people around her are afraid of negative triggers. But this form of cutting isn’t attended to kill themselves. When I talked to my friend she knew exactly what was needed to cut ones wrist correctly. Instead of sorry slashing herself permanently she liked to seek the danger zone to confirm herself she had control over her life.
She went trough various stages during her puberty, also went for professional help. That gave her medication, she became like a zombie all emotions were numb and it made her slow and sleepy. So she stopped taking it. Now she regrets cutting herself, the scars are not pretty. But she snapped out of it. Full of life and even I had to adjust to it, I was full of disbelieve thinking this was all an act, to make me stop worrying about her or something.
But now almost 8 years later I can seriously say she did snap out of it, and goes trough life in a normal fashion. with normal ups and downs. So in a nutshell;
Support her but don’t provide her (tricky, it’s hard to know what upsets her, most never talk). And hope for the best that she doesn’t cut herself too deep on accident (Most known casualties were accidents).