How do you respond nicely to a mean person?

If you're dealing with someone on social media, I always recommend not engaging. Mute the person from your feed. This person obviously is looking for attention and for a fight, and it's not worth getting upset over. If you're dealing with someone you know in person, try to pause and collect your thoughts for a moment, and then let them know that what they said was upsetting to you, that you value your friendship, and you want to figure out how to avoid it happening again in the future. Focus on how it made you feel so they don't feel like you're accusing them.