That reminds me of Age of Wonders when players would disband battle rams near ally towers so the ram would conquer the tower. Then the player would come behind and take the tower, and the AI would say nothing about it if it were a neutral tower
.
In the example that you give, I don't think there is a black or white answer. It is probably settled on a case by case basis by the players, and both can probably give you a good reason for why it should or should not happen. Personally I would end the NAP and take my land back, in addition to some of theirs.