Why was she on the rooftop?

Upon hearing Bianca's words, Donald became angry. He hated seeing Bianca uncomfortable. Even during the times he played pretend with Tabitha, it was known to any observant person with way too much time on their hands to study others that he doted on her.

To people ignorant of the true situation and knew he was engaged to Bianca's sister, most would believe Donald was just really fond of his future sister-in-law.

Tabitha had thought the same, as it seemed like everyone in the family started to have unusually good chemistry with Donald.

Perhaps if Tabitha hadn't been so blinded by love for her family and fiancé, she would have noticed some peculiar patterns and might have caught signs of Donald's cheating earlier. But, oh well, one couldn't really blame a woman who had been so wholeheartedly in love; it blinded her to not notice anything wrong.