Having passions is a good thing. I would be concerned if: she's not able to let other people take turns in conversation, she's neglecting health or sleep, she won't take a break to join in family activities or to see friends, or if she's not getting adequate exercise.
Rather than getting in a position of cracking down on her obsession, I would suggest attempting to gently broaden it and use is at as a means to connect with other people. Perhaps you could take a tour of the costuming department of a local theater or look at the history of fashion.
