Personally I would go back to your client and stress that I would rather collaborate but that multiple attempts to contact them have failed, ask the client if t

Personally I would go back to your client and stress that I would rather collaborate but that multiple attempts to contact them have failed, ask the client if they would be willing to act as an agent to foster collaboration. If the client agrees and gets rebuffed then you can go back to the client and have them kill the contract. This way the client sees that you acted in good faith to provide your client with the best possible scenario and that it was the other company that would not play ball. At which point the client will fully understand that you have to protect your interests and tried to uphold his interest in doing so.