Why would she be taking stock to MAC for verification if she bought them direct from MAC? There is nothing worse than a counterfeit seller that lies about 'getting them direct from MAC' - unfortunately this is very common...
Either someone/a collection of people have been returning fakes to MAC, and the MAC employees are putting them back on the shelves instead of binning them, or this is complete bull. I know which one is more believable!
This is sad news, i hate hearing about 'authentic, trusted sellers' having counterfeit stock. Maybe the fakes are so realistic that these sellers cant tell the difference - and thus stand by their convictions.
I would understand the odd fake one, but 4+ out of 6 is not on. I would file a dispute with paypal, and write to/send products to MAC to get verification. I would also send photographic evidence supporting your claims to the seller in the paypal 'contact seller' section, as the messages to and fro help paypal (and their parent company: ebay) decide who to award the claim to.
Ive heard a lot of counterfeit sellers brag about MAC makeup artists saying their stock is authentic (ive heard 'my MAC makeup artist friend has checked them/gave me them' etc), and there has been at least one circumstance on here of a MAC MA telling someone that a set of blatant fake pigments were authentic (when they were taken to the store for verification). So like the seller said, MA's dont always get it right.
I hope you get a full refund for these (original cost plus original shipping). An ebay auction is a legal contract, and by selling counterfeit goods this seller has broken that contract (and thus you are entitled to a full refund, regardless of what it says in the listing).
Good luck, and let us know what happens