Hi Nicola,
You are an awesome friend for wanting to support her and reaching out for help so you can do this.
As Ani said, don't tell her to stop because chances are she will not want to or she will not be able to.
SH is addicting and so in the process of her quitting, she probably will make mistakes and keep hurting herself.
If she does not currently talk to anyone, maybe you can encourage her to speak to a school guidance counselor about the anniversary of her dad's death. I know from experience that getting through the anniversaries of those we love that have passsed away is really difficult and she can get help with how she feels emotionally around this time and any other issues she may be going through.
Keep encouraging her and letting her know that she is not alone and that you are here for her and let her know she does not deserve what she does to herself.
Also, if you would like, you can give her TeenHelp's list of
self-harm alternatives They are very useful and they may not all help her but they are a great place for her to start finding distractions. My personal favorites are the fifteen minute rule and homework (I get a lot done with those.) Not sure homework is on the list though.
Hopefully this helps and you can
PM me if you ever need anything.