Adding chokes sounds great EXCEPT...
The gun is now, today, illegal, and the owner, now, today, is committing a crime. And any gunsmith would commit a crime by taking the gun in and by working on it. Technically, even those who know about it and don't report it are committing a crime.
The owner can't even destroy it, since that would be destroying evidence of a felony, the felony being owning the gun in the first place.
Legally, the only thing the owner can do is to turn the gun in to BATFE or the police.