Is secretly or quietly describing women as "whores" due to their immoral actions considered backbiting, and does backbiting apply to non-Muslims?

What these girls are doing is forbidden (haram). It is your duty to advise them, enjoin them to do good, and forbid them from evil through lawful means. It is not permissible to describe them as prostitutes, because prostitution (al-ʿuhr) is unlawful sexual intercourse (zina). Their merely talking to boys and engaging in conversations that lead to indecency does not constitute actual unlawful sexual intercourse, even though it is metaphorically considered a form of unlawful sexual intercourse, based on the Prophet's (peace be upon him) saying: "Indeed, Allah has decreed for the son of Adam his share of unlawful sexual intercourse, which he will inevitably attain. The unlawful sexual intercourse of the eyes is looking, and the unlawful sexual intercourse of the tongue is speaking. The soul wishes and desires, and the private parts either confirm that or deny it." Therefore, it is not permissible for you to say that these girls are prostitutes. What is required of you is to advise them and invite them to good with kindness.