I think the one thing that's been overlooked in regard to not saying anything is the potential humiliation her friend could face at the hands of someone who's not so kind. That being said, if approached in a delicate way, it could be the kindest thing to let her know that you know she's carrying a fake. If she wants to keep carrying it, that's her business but if this is a dear friend that I don't want to see hurt, I'd strongly suggest to her that she quietly carry her fake bag.
Just my 2 cents worth.
Just my 2 cents worth.