18 Pa.C.S. § 506: Use of force for the protection of other persons.

(a) General rule.--The use of force upon or toward the person of another is justifiable to protect a third person when:

(1) the actor would be justified under section 505 (relating to use of force in self-protection) in using such force to protect himself against the injury he believes to be threatened to the person whom he seeks to protect;

(2) 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

(3) the actor believes that his intervention is necessary for the protection of such other person.

(b) Exception.--Notwithstanding subsection (a), the actor is not obliged to retreat to any greater extent than the person whom he seeks to protect.