I agree that it was very strange for Haein to laugh. I don't think it's funny. It could have been funny if we hadn't seen him being attacked so viciously. That father acted like a psychopath. Maybe if he had pelted him with rotten eggs or something it would have been amusing, but a savage beating = not funny. A mitigating circumstance is that Haein did not see the tape, so she probably imagines something comical, like her dad yelling a lot and maybe shoving Sukhyun.
What I do think is funny is that everyone kept saying that "his arm fell out of his shoulder." Too bad the subtitler didn't know the term "dislocated."
I think after viewing tonight's episode, I think it's safe to believe that HI's feelings are definitely changing as in the way she sees KW. She was there for him as a friend today, but there was something about the way she was looking at him that says there's something else brewing.