Pennsylvania law allows for the use of force against one person to protect the safety of others as a defense to prosecution for assault crimes under specific circumstances:

the actor would be justified in using such force to protect himself against the injury he believes to be threatened to the person whom he seeks to protect; under the circumstances as the actor believes them to be, the person whom he seeks to protect would be justified in using such protective force; AND the actor believes that his intervention is necessary for the protection of such other person.