It happened quite famously
to Patty Hearst
when she was kidnapped
by the Symbionese Liberation Army
Stockholm Syndrome occurs whenever a person is in the presence of, and cannot escape, a person who is temporarily more powerful than he or she is, and is threatening in some way. This can happen from kidnapping and rape, and also from plain old ordinary abusive relationships.
The captive is forced to concentrate on the actions of the captor, and they begin to find ways to appease the abuser as a way of avoiding further abuse. Through a form of cognitive dissonance, they begin to focus on good things about the aggressor, and slowly begin to identify with them. Later, when/if the victim has survived the trauma, they still may identify with their former tormentors.
It's a bad idea to reintroduce a sufferer of Stockholm Syndrome and their former abuser, as the victim may regress to trying to gain the approval of the abuser once more. The best way to treat Stockholm Syndrome is to try and bring out the repressed anger the victim still feels towards the captor. Not only will this be theraputic, but it will also help the sufferer avoid more such encounters in the future.