I think they already tipped off the elf girl using some kind of magic communication to let her know they are coming to save her and the younger girl. I'm guessing that her attack was an act to help him look good and earn their trust. The slaver did make a comment about being surprised that she got out of her restraints and was able to use her magic. Of course, I could be way wrong since I haven't seen any raws or read this story before. But seems like something a clever kid like him would do.