I agree that ultimatums rare work, but I don't think that's what it was (the final walk-away).

When she threw away the keys, that was confrontation; showing the individual that there are consequenses to lies. Remember, they were in group therapy and she was the Group Leader. In Group, you make a promise to always be truthful to the Group. They can't help you if you lie.

The walk-away in the parking lot was just that. She had decided that the father's hold over John was greater than his love for her. In lying to her about whether the thugs were right in thinking his father wasn't dead, showed her that John was reverting back to the old patterns of lying and concealment of truth.

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charlie98210
"Whoa!" she said. "Cry 'Havoc!' And let loose the dogs of war!"
And, like Banquo's ghost, they just won't go away.