Here's an extract from the section that @SteveMiller referred to.
Defending Others
The situation becomes even worse if the state you live in adopted additional rules for defense of others. In these states not only must you meet all five principles of self defense [Innocence, Imminence, Proportionality, Avoidance and Reasonableness], but also all the additional factors they've piled on.
In Delaware, for example, if the person you rescued could have retreated, then you must try to get them to do so before you can defend them. I struggle to imagine how that can easily be done in a high stakes situation. And even if the required attempt is made, what evidence will remain of that attempt?
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Here's the DelCode:
§ 465 Justification — Use of force for the protection of other persons.
(a) ....
(b) Although the defendant would have been obliged under § 464 of this title to retreat, to surrender the possession of a thing or to comply with a demand before using force in self-protection, there is no obligation to do so before using force for the protection of another person, unless the defendant knows that the defendant can thereby secure the complete safety of the other person.
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So, a bit of caution is necessary. Branca says you must try to get the third person to retreat but the law says you must know that retreat will secure complete safety. Either way, you must consider retreat. Of course, if it is grandma using a walker, your options are limited.
As a side note, the author's less desirable personality attributes, as demonstrated in an earlier thread, are not visible in the book's verbiage. I recommend the book.