After you leave Candlekeep, you are tasked with making your way to Friendly Arm Inn. Leave the first wilderness area on the east side for another wilderness area further east. Move and leave north in the new wilderness area to be able to reach the Inn. Once there enter the keep and as you approach the entrance to the Inn you will be attacked by another assassin. Kill him and enter the inn. Khalid and Jaheira are in the northwest side of the first floor of the inn.
After speaking with them this achievement will pop.