This is a criminal case, most likely. A warrant is issued when people do not appear in court when they are supposed to. A warrant is authority for the police in that other state to arrest him, but not authority for the police in New York to arrest him. If a warrant is "out" for him in another state, he can be arrested on the spot in that other state if he enters that other state and happens to be stopped by the police.

If New York has been notified that there is a warrant for him in another state, this is the usual scenario:

1. The NY police won't go looking for him on the out of state warrant, because there is no warrant in their state.

2. But if he happens to be stopped by the police, and they check on him, they will find out about the warrant in the other state.

3. They will often hold him, pending an order of extradition from the other state.

4. Extradition is usually only ordered if it involves a major crime, because the state that wants the person has to pay all the costs of coming to get the person and taking him back.

5. If no order of extradition is issued in a reasonable length of time (2 weeks?), then he will be released.

The letter to appear doesn't change anything. If there is a warrant out for him already, not appearing in court won't add anything more to it.

If he wants to clear up the charges, he needs to go to the other state, turn himself into the police (who will take him into custody and hold him in jail) then appear before the judge on the charges.

Hope this helps