Fine, you're not profitting from it directly, so I won't sue, but it would piss me off to have someone pass off *my* creation as their own. Just give me some friggin' acknowledgment. It's not a costume for a Halloween party, it's her damn job that she's gonna put on her resume & certainly people may hire her based on her Harry Potter work. If the The Weird Sisters covered a U2 song in the film (for example) & the film's composer tried to pass it off as an original creation, I'm pretty sure the "It was just for a movie, chillax!" argument wouldn't seem so reasonable.