Well I think I would tell her gently 'cause if she gets made fun of (which is so ridiculous, but I remember HS) I would feel bad that I didn't say something before hand. Was the person who gave it to her a close family member or someone really close to her? I don't know, I might say something like, "I know you got that bag as a gift and it was so generous of someone to think of you, know you like bags, and pick something out for you but I have to tell you I don't believe it's an authentic LV. I really wanted to tell you before someone else, like a stranger or someone at school, decides to say something unkind."
Thats the most gentle and straight forward way I could really think of how to tell her.
Good luck.