I have lived in NYC my whole life and when it comes to dealing with how unpredictable mentally ill people is, it's best to analyze the situation before playing Superman.
Once he grabbed her hair, I would not have done anything either but become clenched up, watched him, and be ready if it escalated any further.
You don't go charging in situations in which you can make it 10x worse, especially when dealing with mentally ill people high off drugs. You have to also protect your life as well. These the same nikkas who can withstand tasers or bullets.