Except said hot ninja girlfriend had never defied his sister (to his knowledge) and, in fact, had been known to follow her instructions over Zuko's wishes. He had every reason to think she would be horrified that he was turning traitor and turn her in. And even if she didn't, he would have put her in a terrible position. Either turn in her boyfriend or be party to treason. I think not telling her his intentions beforehand was the more noble option in this case.

Although I would agree with the "no text breakup" as a general rule. :)

I'll post my answers on my journal.