You have to decide for yourself if you're going to plug it or not because if it all goes wrong no one else is going to take the blame
I'm of the opinion that tyre fitters/vendors/manufacturers will always take the safe option of telling you to replace it. Firstly, they can't tell you to plug it if there is any danger to yourself, and secondly, either you or someone else will sue them if something happens.
Personally, since it's only a nail and the hole will be quite small, I'd plug it. This is not me telling you what to do, I'm just telling you what I would do. Incidentally, I just pulled a pop rivet end out of my tyre yesterday, luckily it went in sideways in a big tread block and didn't cause a puncture.