Is a friend's complaint about her problems with relatives (such as her sister or husband) – for the purpose of relief or consultation – considered backbiting or calumny, and does the same apply if the complaint comes from me to her?

Backbiting is a major sin, and it is not permissible except if it is the only means to achieve a recognized benefit or avert an anticipated harm. What transpires between the questioner and her friend, in terms of seeking advice and counsel, is permissible, provided it is limited to what is necessary and does not involve disclosing what passes between them.