Would it be considered backbiting or calumny if I talked with my sister about the negative aspects of a particular friendship and told her of my desire to distance myself from it, without directly mentioning the friend's flaws, and what is the expiation for that, and what is the supplication that protects me from bad companions and undesirable people?

Forbidden backbiting is to mention a Muslim with something he dislikes in his absence, whether it is concerning his body, or his religion, or his worldly affairs, or himself, or his character, or his property, or his children, or anything else related to him, by word, or writing, or symbol, or اشاره.

Stating the harm of befriending a girl is not considered backbiting if nothing related to the girl is mentioned. Likewise, mentioning a girl with what is true about her for the purpose of advising and warning against her company is not forbidden backbiting; rather, it is one of the reasons that permit backbiting, such as warning and advising Muslims.

Slander (namimah) is conveying speech between people to cause mischief, and the questioner's action has no relation to it.

If forbidden backbiting occurs, one must repent to Allah.

A Muslim should pray to Allah for righteous companionship and seek refuge in Him from bad companionship, based on the saying of the Prophet, peace and blessings be upon him: "O Allah, I seek refuge in You from a bad day, and from a bad night, and from a bad hour, and from a bad companion, and from a bad neighbor in the permanent abode."