if it's affecting a person dear to you, be very mindful about the subject and treat it with discretion. It can be threatening to them to have to confront the topic directly.

https://www.nami.org/co-occurring-conditions/self-harm/

edit: also, if the cutting isn't recent, if it's already all old scars and no fresh cuts, take it easy, maybe the kid is stable, might already be learning to deal with their issues in a different manner. Just providing a safe and trusted environment means a lot.