What is the ruling on alerting a woman whose 'awrah has become partially uncovered, such as her hair or neck, and how should this alert be given?

Enjoining good and forbidding evil is a collective obligation (fard kifaya). It becomes an individual obligation (fard ayn) if a person is the only one capable of changing a wrong, and it is obligatory according to one's capacity. So, if you are secure from temptation for yourself and for the woman, and you do not fear a greater harm, then you must alert the woman to cover what has been uncovered of her. However, if you are not secure from temptation or you fear a greater harm, then it is not obligatory.