I would say any kind of jewelry would work just fine! Girls love getting jewelry.
Or, since y'all are French, how about a bottle of perfume? All the French girls I ever met had loads of it (they're the ones who got me started on it, too--blame them!), but I'm talking a really expensive kind, like that Lolita whatever...I can't remember the name of it, but it's in the bottle shaped like an apple. Or hey, what about a bracelet to match that necklace? Or a tennis bracelet with sapphires or rubies or some other pretty rock in it? Just don't get her a vacuum or anything like that, and you'll be just fine!
I'm full of ideas tonight.