Someone mentioned it in a comment, but I'll put it into an answer: the person is "speaking from preconceived notions".
Prejudiced is based on pre-judging, but I think it's not what you want. You can be, for example, racially prejudiced and yet have met people of the other race. The pre-judgement is not on a racial basis, but on an individual basis: you are judging an individual without having met that individual, based on what you perceive about the race to which they belong.
Another option that hasn't been mentioned -- but I also think it does not work in your situation -- is opinionated. Opinionated has the connotation of strong opinions not based on facts, but also does not imply this is because of a lack of exposure.