This seems to come up every few weeks on this board.
I'd never tell someone their handbag is a fake. I wouldn't want someone lecturing me about how my skirt is made in the Dominican Republic and how that hurts the US economy or how I should be driving a hybrid or how I bought a new pair of shoes when I could have fed a third world country and rescued kids from sweatshops with that shoe money.
If someone asks me why I don't buy fakes, I tell them. If someone asks why I don't eat veal or buy fur, I tell them. I never try to, even tactfully, tell people about their clothing or accessories unless they ask me.
For your specific example, IMO, telling a friend who is happy with her fake birthday sunglasses that they are fake and they support sweatshops, etc is the equivalent of telling an overweight person that she shouldn't enjoy an extra slice of her birthday cake because she may get diabetes, heart disease, etc and it makes all of our healthcare costs go up. It would be rude, and it would make you a killjoy, in my humble opinion.