My friend is creepily obsessed with Justin Bieber?

I agree. Your friend needs some other interests- like a hobby. It's normal for teen girls to have a crush on a celebrity but it's not so great to wrap up everything in that. Encourage her to shop for decorating items that are cool but aren't Justin Bieber related. Suggest you go do other things, like ride roller coasters, go to the beach, play sports, listen to other music. In time she'll grow out of it- but I agree- not too many people will find her interesting if that's all she ever cares about.