She was a strong character but that doesn't mean she wasn't emotionally vulnerable or unable to be compromised, Personally I would have preferred Ben becoming Caedus Sith apprentice-imagine the psychological torment that would put Luke through. Though come to think of it-I'm really that sure how psychologically robust Tahiri actually was. She was brainwashed by the Yuuzhan Vong after all and developed the personality of Riina. She was also if I recall an orphan who lived amongst the tuskens on Tatooine. Her relationship with Anakin seemed to pull her in a stronger direction and his death did affect her greatly. So it's clear I think when you look at Tahiri's biography and then look at her psyche-she wasn't that strong at all and was in fact quite vulnerable to Caedus pyschological and emotional manipulations.