[Blueface]

Well,
Assuming by authentic Partagas, you mean the CC kind.
Given that island is of French origin, we know they can have them to sell if they wish.
Now knowing if legit or not? Loaded question with too many answers.
As to spotting fakes, quite complex of a response involved to be sure you hit all points.
If he is there regularly, tell him to try to find out the word on the street on the vendor he intends to use.