Will a person be sinful if they seek the help of another to change an evil and lift an injustice that will befall several people, and they expect the perpetrator to receive their just deserts, and their intention is to protect her and prevent harm?

Backbiting is originally forbidden, but it is permissible if it serves a benefit or prevents harm, as mentioned by Ibn Daqiq al-Id. This includes seeking help to change a wrong (munkar) and guide a disobedient person back to righteousness, such as informing someone capable of removing the wrong. Therefore, there is no sin upon you for seeking the help of your friend's relative to prevent her from committing a wrong, as long as it does not result in a greater harm, such as assaulting her. As for merely treating her in a way that prevents her from falling into wrong in the future, such as restricting her outings, this does not incur sin.