Again, proper planning and communication is the key. If there was not a specific 'what to do if we are seperated plan' then you both share the blame. If you do not Plan the Dive, then it is impossible to Dive the Plan. Refusing to talk about this will not make him a better diver, nor will being PO'd at him (not to mention what it can do to your blood pressure). If, after calmly and without fingerpointing, he refuses to acknowledge that you should have had a plan, then you can and should (IMAO) refuse to dive with him any more. Assigning blame is not the point, realization of what went wrong and what steps we can take to prevent reoccurance is the lesson to be learned. (again just myAO)
Happy and Safe diving to you,
Matt