Could just be that he was smiling at her when he said "i would, but you've been a bad girl", that might have been enough to convince her climbing was her only option.
When he forced her out there, he could have shoved her right up to the edge. That, coupled with all the comments about the balcony during the evening would be enough to scare the crap out of someone. If he stood there and smirked when she was crying and begging to go home I am not surprised she climbed. She probably figured one way or another she was going over that edge that night.
At what point did the neighbours actually come out and look? It was only after she was already hanging there, right?