how it usually goes in most game is:
- Your old world will still work but to get the new stuff you'll have to generate a new updated world.

So if im right, your friend will still have his castle but the new stuff will not be in his game.

Also, he can install a blueprint mod allowing him to create a blueprint of his castle which he will then be able to place in the new updated map and simply throw all the mats in there until completion. (meaning he wont have to redesign his castle, only farm the materials to spawn it)