(a) A private person who makes, or assists another private person in making a lawful arrest is justified in the use of any force which such person would be justified in using if such person were summoned or directed by a law enforcement officer to make such arrest, except that such person is justified in the use of deadly force only when such person reasonably believes that such force is necessary to prevent death or great bodily harm to such person or another.
(b) A private person who is summoned or directed by a law enforcement officer to assist in making an arrest which is unlawful, is justified in the use of any force which such person would be justified in using if the arrest were lawful.