If you are ordering in Polish
He's from Poland, however if he's getting this problem due to accent issues caused by a long time away, he would do well to specify that he wants one, not any other number.
Even when I was fresh off the boat with very basic language skills, at a time when most Poles hadn't met a foreigner, I never had this. I do remember a few barstaff clarifying how many the order was for.
Of course it could just be the sort of bars he's going to.