I think it would be virtually impossible to win on the very hardest difficulties because even if AIs are initially only slightly friendly with each other, it could very well be that once one one AI declares war, soon all of them will. Ridiculous and Insane should be extremely difficult, but not impossible.
Generally when the human declares war, they are ready for it. It is not really so much of an advantage when an AI declares war on the player. The AIs are likely enough to declare war on the hardest difficulties even without the helping a friend bit, because they have a lot bigger empires.
So, what do you think about instead of AIs having a default two-way non-aggression pact, they only help their friend if the player was the one to initiate hostilities or refuse to get out of territory? This would provide an interesting strategic element, and would also discourage the player from being the one to go around starting wars.
What do the devs and other players think?