This is unlocked once you've reached the city of Baldur's Gate.
Abela is being held captive by a mage called Ragefist. His house can be found at the top of the screen in West Baldur's Gate.
Just enter the house, kill him and choose the dialogue option to set her free. There are no achievements linked to any of the other options that come up.
The actual quest to release her (The Captive Nymph) can be gotten from Ramazith in North Baldur's Gate or from Lady Hannah in Central Baldur's Gate.