Yes but who's to judge what is "an disturbance of enjoyable game play"? For his allies the fact that he quits the map is not enjoyable. And is increasing joy if he gives them his territory.
I personally am of the opinion which I expressed before that if they worked weeks together and for some reason one of the allies need to quit that it's not a bad thing to do. And for the enemy it's easier to fight an army that dropped moral then when the ally would have stayed and fought on. However if this happens early game and the 2nd person never played on the map I find it a rule break. As with many things context is key.