Great advice above. Just a few extra notes to add. I think the most important part is to emphasize that you are there for her whenever she is hurting--no judgement whatsoever. Aka offer your shoulder for her to lean on when a bad mood hits. I know when I'm with people it helps a lot.

And eventually offer up other ways that aren't as harmful as SI. And less permanent. Like most addictions, it's hard to break but there are a lot of other things that can help keep your hands busy and your thoughts focused on something else. I find it soothing to make tiny paper origami. Something more similar to SI though is:
-rubbing ice on your area of choice
-snapping a rubberband
-drawing the line with a pen or pen cap

But the most important part is to be loving and understanding.

Best of luck and keep us updated!