At some point in your journeys with Nalia (after you've liberated her father's keep), a messenger will come and tell her to come to her father's funeral in the Graveyard District. Once you go there, she will meet her old "friend" Isaea Roenall who is still upset with her. He won't cause you any trouble yet but the next time you visit De'Arnise stronghold he will arrest Nalia and bring her with him.

Shortly after, one of his soldiers will approach you and tell you how you can compromise Isaea's career and reputation. To do so, you'll need to talk to Barg in the Dock's district and discover that Isaea is practicing piracy, you'll then need to see Officer Dirth in the Sea Bounty and get a scroll from him talking about Isaea slaver activities and you'll need to enter Isaea's estate (the Roenall estate) in the Government District to get a book that talks about some gem smuggling. After you have done these 3 things, you can go and see Isaea's superior (Corgeig Axehand) in the Council of Six building in the Government District. He will arrest Isaea and Nalia will be released.

On an important side note, the Slavers that Roenall was trading with are revealed in one of the documents recovered as "basing their operations from the guarded compound" in the Temple District (many people overlook this, and it finally presents a quest link to the Temple Slavers and their coveted treasure).