Even if Harry and his friends hadn't told Bones that there would be trouble he was going to expect it anyway. The Death Eaters tended to strike when people gathered and Halloween was certainly full of gatherings. Parties, trick or treating, and everything in between would draw the murderous assholes to Muggle towns to try and complete their genocidal mission. Bones wouldn't let them. He was too late to save a few of them, but as soon as the Death Eater raised her wand again, Bones was there with a spell to knock her off her feet and interrupt the curse.
"Get to safety," he told the Muggles nearby. "Now!" This was definitely a girl. Whether it was Bellatrix or another girl he couldn't be sure, but it was a shame to think that there were enough to be confusing one with another.
"Why don't you pick on someone with the ability to fight back, coward?" Bones snapped, sending a jelly-legs hex her way so that she wouldn't be able to stand. He didn't want to kill her. He wanted to take this woman in. Maybe with a little persuasion she would talk and give up some names. He didn't imagine Death Eaters to be a loyal bunch to begin with. He wanted to test that theory.