Kati Morton has two videos that might be helpful. I would encourage you to watch with your son so you can help him process the information. Ultimately what he needs to know is that he can be a caring and supportive friend, but it’s not his job to keep his friend safe from self harm. That’s her job, in collaboration with her parents and therapist. Sometimes teens can get really enmeshed with taking responsibility when a friend is doing self-destructive things. Self harm is usually a symptom of internal struggles and will resolve as those are treated.
I think he’s already been a great support to her by wanting to understand and help rather than being disgusted or shaming.
Self Harm: Why Do We Do It—
https://m.youtube.com/watch?v=gXwYFjy-S6k
How to Help Someone Who Self Harms—
https://m.youtube.com/watch?v=mzUz1pfR41Q