"Just remember," he advised, "what your professor said about the blood status bigotry Theo might have been raised with, especially if it’s true that Mr Nott is some kind of war criminal who escaped justice. Mind you that’s just alleged – not proven. Now, even if young Theo’s father is a bigot, or even a criminal, that doesn’t mean your new friend has to be one too! Clearly he’s at least trying to be open-minded about you being a "Muggle-born’, so it’s quite reasonable to give him a decent chance – children aren’t responsible for the actions of their parents, so don’t blame him for any of that. I’m sure he’s heard enough slander about it already, and none of it could possibly be his fault."