What am I supposed to do? (TW: Self harm)

In the first few weeks of school, I had some people I talked too. They both eventually joked around that they had tried to commit suicide before. It's been a long time since then, and they are both in therapy and have many friends, so I hope they're okay and I try not to worry so much.

Earlier today I was talking to a girl in my class, and she was talking about what she had in her pockets. Something I can't remember, a pencil... and "a sharp object." I had said, "Wait, what?" And she was like, "Yeah from a razor."

As soon as she said that I thought, "Oh shit." I realized what she meant. Then she lifted up one of her sleeves are there was a small scar. "Oh we don't talk about that." She joked before lifting her other sleeve. I saw many small scars.

I don't know what I'm supposed to do? The razor in her pocket proves she still actively self harms. Why would she tell me, a random person that? What am I supposed to do? How do I know she's okay? I'm honestly scared for her even though I barely know her.