You don't say anything. You move on. The key is not listening more than what you say. The real question is what do you say to those who give them a venue. Clearly someone at the UN made the decision to invite her to speak. Maybe they deserve persuasive feedback about how that undermines the perceived seriousness of their organization. I say persuasive because there is a risk at making it sound like they got what they wanted. It's not Greta's fault that she as a sixteen year old was invited to speak at the UN. What do you expect her to do. She's allowed to have crazy misinformed ideas. She's sixteen. It's not her fault she's being listened to.