I don't understand why people say it would've been nicer of Lieze to just tell Isaac that she's not interested in him. As long as Isaac doesn't tell her about his feelings toward her, she has no way of telling him she's not interested. Furthermore, she doesn't have a responsibility to tell him for the same reason.
If anything it's kind of her to clarify to Isaac that she's not interested in him in this indirect, yet effective manner. She didn't have to do anything, as he didn't actively pursue this romance.
The translator's comparison also goes flat. Sending an e-mail/text to someone means you're directly telling them how you feel about this. Lieze managed to convey her position without even confronting Isaac.
TL;DR, Isaac isn't pitiable. Lieze handled this as effectively & kindly as she could in her situation, as Isaac didn't leave her any other options.