What do you mean by "my co-worker sells MAC"? Did she sell you a single product that she didn't like or had a double of or whatever, or does she regularly sell makeup to people around her?
If it is the latter, she more than likely knows. It's MAC, not Avon. You can't just open up your own little business and sell it.
If people want to buy fakes that they know are fakes (this goes for all labels and products), that's their decision as far as I am concerned. But if someone is selling fakes and claiming they are the real thing, not to mention, taking the price of the real thing for them, I have a big problem with that.