That's why the rule is unwritten.
It all depends upon you, LadyEsme. Do you want to remain friendly with the person who's npcs you want to farm? If you do, leave his npcs alone, or make sure he doesn't suspect you ;)
Just make sure you are ready for the repercussions if he does. He may decide to retrieve the resources directly from your stores. Or his alliance may declare on yours. It wouldn't be a first.

