When are you allowed to use force to defend someone else?
Defense of Third Person
A person is justified in using for or deadly force against another to protect a third person if:
1) under he circumstances the actor reasonabley believes them to be, the actor would be justified in using force or deadly force to protect himself against the unlawful force or unlawful deadly force he reasonably believes to be threatening the third person he seeks to protect; and
2) the actor reasonably believes that his intervention is immediately necessary to protect the third person.