John Stuart Mill on Should Yuki share her friend's mental health struggles to get them help?

Yuki and Darian have been best friends since ninth grade. Last weekend, Darian broke down and told Yuki things they have never told anyone: that they have been struggling with severe depression for months, that some days they cannot see a reason to keep going, and that they have been thinking about hurting themselves. Darian begged Yuki not to tell anyone — not their parents, not the school counselor, no one. "You're the only person I trust," Darian said. "If you tell someone, I'll never forgive you." Yuki is terrified. She promised to keep the secret, but now she cannot sleep at night worrying about Darian. She knows that breaking her promise could destroy their friendship and violate Darian's sense of control over their own story. But she also knows that if something happens to Darian and she did nothing, she could never forgive herself. Yuki is caught between respecting Darian's autonomy and preventing potential irreversible harm.