Boss hunting has a -15 to your skill, that other jobs at the guild do not have. That still means you had 52% to succeed, so you are definitely unlucky. But maybe she did multiple attempts, and was one-shotted in one of them. Try to reach more than 25 HP before sending someone to fight a boss, and use the "daily story limiter" in manage buildings to only do one attempt in that building, that will avoid all death scenarios.