You're looking for chapter eleven, Hermione's Helping Hand.

Quote Originally Posted by Half-Blood Prince Chapter 11
There had been a horrible incident the day before, when Hannah Abbott had been taken out of Herbology, to be told that her mother was dead. They had not seen Hannah since.
That's it. That is Hannah's entire appearance in HBP, and it's only a report of the previous days events. We don't actually see her again until the end of DH. She isn't reported as being in the Room of Requirement, but she's guarding a tunnel with Lee and (Fred? - it might be George!).

As for her personality, she seems to be bervous during her exams, and rather quiet. As she later becomes a pub landlady, I don't think that she'd nurse a grudge, she'd certainly forgive minor misdemeanours. But I can't see her forgiving her Mum's killer.

-N-