You will bring your motion in court, and properly support it, establishing that the lawyer was not authorized by the person who owns the debt to file the lawsuit. If the court requires an evidentiary hearing, you will attend your hearing with actual evidence and witnesses to present in support of your argument. In the alternative, the named plaintiff can come to court and petition for dismissal.

There may be other or better approaches depending on the full facts, and it would seem that you're leaving some important facts out of your narrative.