If he's had it on there that long then it will be rusted on,the mod will retain moisture after shooting,and,the thread is bare metal so will rust as quick as feck.I know someone that had the same problem,and,I think he put the mod in a vice in the end with the aid of a gunsmith.If you are going to put anything down the barrel to try to release it then I would suggest 'Kroil' it is a very good creep oil,and after a few days of dripping some on the barrel to run down onto the thread it might/should be a bit easier.......good luck though it won't be easy.
Martin