With console commands, you could pause, play as your ally, negotiate peace, switch back to your empire, and unpause. 

There's no legit way to do it, though.

I'm currently in 3 entire wars together with an AI empire that I have a defensive pact with. 2 wars are still pretty new, but in the first we have 58% warscore, yet my AI ally who is war leader will not negotiate peace. Is there anyway you can force the AI to make peace or deliver some kind of ultimatum to your ally that you're about to peace out yourself?

Can you make the AI declare peace?