Just because the location is open to the public does not mean that the employees may entertain guests while working.
My younger son works in retail (actually to make ends meet, he works two jobs). My wife and I try to meet him for lunch or dinner fairly often, both to be with our son, and take on a small part of the cost of his meals.
We do not stand around his counter and talk to him. We usually wait in the parking lot and know that he has to finish helping his customers, or other assigned tasks and be told he may go to lunch before he comes out. This is a public area, but if he spent time talking with us, even if he continues his job, it would not present a favorable impression to the customers that walk by. His employers have a right to expect that he gives a favorable impression for the store.
But, you indicated that this was a “year end work do”. I would think that if guests were allowed, you could bring whomever you wished. The only caveat that I can see here is that if your boss and your companion already can't get along, (and can't be adult enough to grit their teeth and act cordially) it might be better not to aggravate the issue,
As for the part about harassment, if he is persistent, or if a single instance is egregious, this is a matter to be taken higher in the company (if there is anyone in higher authority).