Board Thread:Dark Souls II/@comment-24129583-20140528041702/@comment-6003396-20140529073041

I only ask because I've heard of a glitch where, even after giving her clothes, she won't enter inner Majula but instead stays where Bernhart is found, which is where she is if you don't give her clothes. If Boss is affected by the glitch, then it might explain why he isn't getting the achievement.

Other than that, though, I can't think of any reason why Boss isn't getting the achievement. It might work on a different playthrough.