Ideally you'd know this person and interact with them somewhat regularly. If it's online or this person believes they can ignore you without consequence then it won't matter how you address them beyond that initial interaction.

Don't use that as an excuse to feed into their believes though. There's a chance someone they know is already trying to coax them back to reality. If anything in that interaction makes them feel justified, it'll take 20x the effort to undo the mental hoops they'll jump through to rationalize the rest of their feelings.

It sucks. It's not fair to those impacted, but this is the only way to stop this radicalization and hardheadedness that plagues people nowadays.