First, you need to take the stairs down the hidden area Samand has just revealed and tell the guard you have permission to talk to Mateo. Speak to Mateo and he will say that he also suspects Kiser Jhaeri, and that the countess gave a false testimony.
After seeing Mateo, go to the countess' house and speak to her. The Countess willingly enough tells you that she gave false testimony because Kiser Jhaeri has abducted her son Ardic.
Next, go to Kiser Jhaeri house and speak with him. Kiser admits having abducted Ardic but claims it was to protect himself from being falsely accused of treason. He says that the person managing the defense of the ctiy, a wizard named Errard, is the true culprit. Kiser asks you to kill Errard for him. (If you are Evil, Kiser recognizes you as such and admits to being the traitor. He suggests that you could kill the Countess instead. If you do Jaheira will leave the party).
Go and find Errard. If you kill him you will lose a lot of reputation and the entire city militia will turn hostile. Rather, speak to him and tell that Kiser Jhaeri said to kill him. Errard will cast a divination spell to reveal Ardic's location, finding out that he is being kept in Kiser's home.
Go back to Kiser Jhaeri's home, and Kiser will flee downstairs. Before following him, prepare for a large battle with thieves as well as a high level mage. You will find a hidden door in the walls downstairs somewhere. Open and go through it, and you will eventually reach Ardic. Speak to him and the party recieves 7000xp. Speak to the countess after and you will receive another 5000 XP and 2000 GP.