It has to be  wide, loose,  modest and covers the body completely. Therefore, if it is covering her waist, this seems ok providing that the skirt is loose. But if her waist and buttocks curves are exposed, this is not permissible.

I know that a Muslim woman can wear any style of clothing in public provided it meets the conditions of Hijab. However, I want to know whether a large khimar or shawl covering head, hair and upper body (sometimes reaching the wrists) will suffice as a jilbab or outer garment, when worn over modest, Islamically-approved clothing such as plain modest-looking, loose and flowing skirts and blouses or an ordinary looking Indo-Pak shalwar kameez?