Well. It happened to me. He said that bitcoin is currency used by terrorists and by illegal markets and...yes, he was right. If someone has a hatred on bitcoins, or, actually on anything, I usually agree with his point of view and then add more over his point of view. I told that guy that he is right, debated a little on that subject, and then told him that it is actually used everywhere as there are less fees and other stuff like that. I've also told him about what I do (trading) and he started to see that it is not something bad, it actually helps people.