Helping a Depressed Friend [TW]

[TW: self harm, depression, suicidal thoughts and actions]

My best friend is not okay. Theyve dealt with a lot of mental health issues lately, and what started as feeling bad has quickly escalated to self harm and breakdowns. Theyve only just started talking about wanting to kill themselves and Im really scared. They keep getting into this headspace where they dont think about the consequences of their actions and just act on impulse. I dont have anyone I can turn to, talking to any adult around me would require their parents being involved and thats not an option here.
I want to be there for them really badly, but I really dont know how. I always feel like Im making it worse. And when they get too descriptive about the way theyre feeling I end up breaking down on them and they end up taking care of me instead of the other way around. I feel useless.
What can I do? How do I comfort someone someone going through this? How can I be there for them? How can I better understand what theyre going though?