Anna and Boris are friends. Boris intended to hurt John by using a rock to knock his head. Boris asks Anna to pass the rock to him, and Anna passes it over. At that moment, Anna is not sure what Boris will do, however, she thinks that it is very likely that Boris will hurt John. Boris then uses the rock to knock John's head and John is seriously injured and becomes unconscious. Then, Anna informs Boris that he has gone too far and she attempts but fails to take the rock from him. Boris continues to hit John's head with the rock. As a result, John dies.

Discuss the potential liability and defence of Anna and Boris with the support of UK case law.