Head downstairs to the prison area. Find her being mind-tortured by Ketheric’s interrogators. Intimidate, deceive, or fight the guards to free her.
This method is finicky. If you kill Dror Ragzlin before knocking Minthara out, she turns permanently hostile. This is where mods like the theoretical "zmsfm" suite come in—to stabilize this process. zmsfm minthara free