Personally as a nihilist, I don't believe in things like "good" and "evil" as I believe they are constructed by people and should they exist they can never clearly be known. That being said (as horrible as this sounds) Hitler had a mom, a dad, a lover, was artistic, and had qualities we would all see as good in any other human being, so to say Hitler was horrible or evil (or to even demonize the names Adolf and Hitler, yeah almost anybody with that last name changed it and rarely has a baby been born into a non-racist family named Adolf) is a gross exaggeration of generally bad things he had done.
However while I won't say the holocaust was "evil" I could never agree with it or its principles of mass extermination of ethnic and political groups. Further, I heard a quote somewhere (and I'm paraphrasing) that I think sums up the Holocaust perfectly. "Intolerance isn't needed to have genocide, only apathy." So while Hitler may have been the one calling for the heads of all the minorities, the German people knew about it, and either ignored it, or took part in it. In summary, it would be like everyone hating Obama if 10% of Americans (with Obama's support) went out massively exterminated illegal immigrants. Hitler as a person was a figure head of the german people who either ignored the injustices, or took part in them. I doubt Hitler ever killed anyone with his hands.
And before anyone thinks I'm defending hitler and labels me an evil person; as an atheist and a socialist, I wouldn't exactly have voted for Hitler, or had a beer with him, or been one of his favorite people.
All in all though, I wouldn't worry about it. It's just dumb teenagers being dumb teenagers, just remember it and 10 years later if you see that person remind them of what they said and see how dumb they look.