I've had to do this with others. Give them one last shake and say "the ball is in your court" then let them wander.

Sometimes, you need to remove yourself from the situation.

Your friend will seek another person to fit the "conspiracy theorist" variable in their formulaic life, and after they've burned enough bridges, they will find themselves isolated. Then they will question because no one will be there to coddle them with information, as they reject this person like a parent.

When this happens, the lefties will turn on them for questioning. At this point it will be your job to remain a beacon, a lighthouse in the storm.

A lighthouse does not grab a ship and force it to land. The ship must sail towards the lighthouse on its own accord. Stand tall and illuminate the path back home, and accept them when they arrive, but they must journey back through the ruined bridges and frayed paths they created.