Even a trained mental health professional shouldn't have to bear this responsibility, and they are released from confidentiality obligations in situations like this. You shouldn't carry this burden alone. You should inform their family members and encourage them to seek help from their doctor or mental health professional, if they have one. If the person gets angry, they will later understand that life and well-being are more important than secrets, that your relationship is based on doing what's best for each other, and that the best course of action in these cases is to increase access to support resources.