This was covered quite nicely at the end of AT, when Spock questions T'Pring about her motives, and after she answers him, he praises her logic in choosing Kirk as her combatant. Her only mistake, was not accounting for human duplicity in the form of McCoy. T'Pring KNEW that Spock was really 'married' to his Starfleet duty, and that whatever happened, she would still have Stonn.
It always irked me that Spock let her off scot free. She was his legal property after the fight. Having heard her explanation and knowing she was treacherous and cold enough to get a man killed, Spock should have ordered her to go live in a Vulcan convent and scrub hospital toilets for the rest of her life.