I agree with Talvalis--- Hiroko knew from the start that Sono had unreciprocated feelings for someone, but she still "took the challenge" and eventually fell in love herself. So, in other words, Hiroko dug that hole herself.
I will say however, that Sono really frustrated me. Sure, I know that she's still hung up on Sato, even though she has Hiroko, but her constant Hiroko/Sato cries really annoyed me because poor Hiroko was so in love. She deserved a proper ending instead of that dumb run-away scheme Sono pulled off on her.
Well, at least Sono taught both Hiroko and Sato how to love. :)