It's all in your presentation of the topic and the vibe you give off. People will forget what you say but remember how you made them feel. Again, something someone is saying online, whether it's /b/ or otherwise, has no relevance to an in person conversation unless *you* bring it up. I would recommend just relating the facts, and doing so in a positive and friendly way. Many aspects of ball earth claims are quite funny, such as the faked moon landing and specious arguments about "gravity". Plus there are deeply meaningful and mysterious implications of the world we actually live in. Think of the ice wall. Think of what's really above and below. It can be very moving and, in the right setting, downright romantic.