Disclaimer: JKR owns Leanne and Zach. I do not earn any money by writing up this crap.
Leanne loves his ex boyfriend, despite everything he has done to her. She was happy, although after being fooled. Even so, she is thankful with him, no matter the humiliation she has to carry upon her shoulders. With the rest of a society that had nothing to do with what she had suffer in her past.
He is just a git that insulted her with knowledge. A git that had gone away, and didn't return in her worst moment. A git that used her to his own needs. And that's why she hates and loves Zacharias Smith more everyday. A little more.