Without an actual picture of the board I am not sure. From a rules perspective you are right, he stuffed it up, its a mishap. I also agree that when he started to place the other minis he had made his decision.
For the sake of the game maybe I would be ok with it, depends on opponent and state of the game. I would try and say before he put down the second mini that he was stuck with his choice. it seems that this arose because you didn't ask about his initial placement being risky. Had you said something sooner his partner may have corrected his error sooner. In practice this is impossible, but I often find its better to state your intentions with grey areas, and agree before the move is complete, especially with LOS issues.