I'm going to borrow from Miss Manners here: it is respectful to treat people as they prefer to be treated. If you know friend doesn't like to have difficult conversations, it is respectful to go by friend's wishes. If you want to probe further, you could put on a puzzled face (lips pressed together, eyebrows slightly furrowed, tilt head slightly, look upwards), and say something in a curious tone. "Huh, that's really strange. How would that happen?" If you sound puzzled and curious rather than defiant, your friends won't take it as a challenge.

Wanted to point out - asking questions may be a sign of respect, but that doesn't mean smiling and nodding is disrespect. Maybe they actually agree with you. ^^ Or if you mean specifically the people who disagree but hide it, there are many reasons a person might not engage in a conversation, and most of them aren't about respect or condescension.