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Well, that is correct. If he is stopped he can be arrested. So he needs to write to the judge with copy to the District Attorney. He needs to request the payment plan and to have the warrant quashed or recalled. He should have it in writing if the Court is willing to do that.
It is possible. But that is the only way he can handle this matter himself.
If he is concerned, he will need to get a local attorney to handle the negotiations for him. That way, his location is not revealed. But the only way to get rid of the warrant is to have it served or quashed.