You may use force to stop their attack on him: i.e. to defend him. So if they are actively attacking, you can hit, or twist an arm, etc.--your response needs to be appropriate and proporational, however; if you escalate the level of force, that is no longer legal defense. (E.g. if they are slapping and hitting him with empty hands, no taking a bat or tire iron to them.)Once they stop hitting, etc. your defense must instantly stop, too: you cannot hit them after the fact because they hit him.