The meeting was purely accidental, but Kelly saw someone who needed help and she couldn't simply ignore it. She just didn't realize then exactly how dangerous Tau was or that helping him could be the death of her. That part only dawned on her after she was taken captive by the aliens raiding for mates and the discovery that Tau didn't want a human mate. This took being 'a fifth wheel' to a whole new, and potentially deadly, level.