My friend cuts and I don't want to see her like this... How can I help?

Does her parents know she does this? How old is she?

Are the cuts across the wrist or along it? Across is usually just a cry for attention, along the wrist (with the vein) is a more serious attempt.

Tell her parents and let them deal with it.

Much self-harm isn’t “attention seeking”, in fact, many self-harm sufferers go to great lengths to hide our compulsions. My scars are all under my hairline and literally no one ever knew I did it.

My personal rationale was that the pain was allowing me to silence my thoughts and focus only on the physical pain for a while. And since I was causing the pain myself, I was in control of that pain and thus I could stop it whenever I liked.

On the upside, in most cases this phase passes fairly quickly – eventually she’ll come to a point where her real-life responsibilities will take enough of her brainpower that her teenaged hormonal overreactions will finally shut up.

On the downside, the compulsion can come back when a person is under extreme stress. I still catch myself compulsively scratching and pinching on really hard days, and I’m 30 now.

Ultimately, there’s very little you can do. You are a good friend simply for wanting to help.

There are some things I read here that are truly upsetting. Running into stuff like this and I suspect myself of leading a rather sheltered life.

If you think or see evidence she is suicidal like your cousin did then there are avenues you can take and that is reaching out to parents, teachers, clergy and suicide help lines. Doing nothing is not what she may need and because you have these tendencies yourself you may want to talk to someone yourself.

Get help. Get attention for this person. You may lose a friend, but they will still be alive. For some, this is not a “phase”. It is a stepping-stone. To permanent.
For yourself: go tell a counselor, a family member, or your GP.
This is Not a release of stress. This is a form of self-loathing and self-mutilation that someone thinking clearly would not do to themselves.
This is not fun or a pain-high. Would you allow your most-beloved, or your child ( future), or your favoite family member go ahead down this path?
The answer is No.
Get or facilitate help.
Please.

Tell her parents, a teacher or counselor at school, or an older sibling if she has one that is pretty much grown. How many of her friends know she does this?