[Q]

I don't think you have to wait for SE V to change this. By editing the AI anger and politics file you can make the AI remain a loyal ally. How was the mood of the AI before he declared war? If it was fiendly it is the politics file you should change, if he was angry, it is the anger file.
On the other hand what you saw is not unrealistic: just think of world war II Germany and Soviet Union.