"You have a fake purse!"

Let her know about this forum. She will find out herself, once here.

Another thing, if they happen to break up, the sentimental attachment will probably end, and at that point you could mention it without fear of hurting her feelings.

LOL, as I read through all the comments to this post, this is exactly what kept going through my mind. If the bag is fake, wait and see which falls apart first, the bag or the relationship.