I hate to say it, but simply yelling "Help! Someone's being attacked! I'm calling 911!" and continuing to draw attention to the situation might be the best thing you can do for yourself and the victim.
Each person has their own level of concious and moral compass. Also, depending on the situation warrants level of force, http://www.kitv.com/r/15076317/detail.html, people were watching and didn't do anything. Hawaii does have a Protection of Others Statute which is http://www.capitol.hawaii.gov/hrscurrent/Vol14_Ch0701-0853/HRS0703/HRS_0703-0305.htm.
http://www.capitol.hawaii.gov/hrs2002/Vol14_Ch0701-0853/HRS0703/HRS_0703-0305.htm§703-305 Use of force for the protection of other persons. (1) Subject to the provisions of this section and of section 703-310, the use of force upon or toward the person of another is justifiable to protect a third person when:
(a) Under the circumstances as the actor believes them to be, the person whom the actor seeks to protect would be justified in using such protective force; and
(b) The actor believes that the actor's intervention is necessary for the protection of the other person.
(2) Notwithstanding subsection (1):
(a) When the actor would be obliged under section 703-304 to retreat, to surrender the possession of a thing, or to comply with a demand before using force in self- protection, the actor is not obliged to do so before using force for the protection of another person, unless the actor knows that the actor can thereby secure the complete safety of such other person; and
(b) When the person whom the actor seeks to protect would be obliged under section 703-304 to retreat, to surrender the possession of a thing or to comply with a demand if the person knew that the person could obtain complete safety by so doing, the actor is obliged to try to cause the person to do so before using force in the person's protection if the actor knows that the actor can obtain the other's complete safety in that way; and
(c) Neither the actor nor the person whom the actor seeks to protect is obliged to retreat when in the other's dwelling or place of work to any greater extent than in the actor's or the person's own. [L 1972, c 9, pt of §1; gen ch 1993]
Thanks for steering me to the statutes. It's interesting reading. (I didn't know Hawaii law gave teachers the right to spank kids, for example!) We can debate what the above means, and how it actually plays out in courts, but there are further considerations when faced with a potentially violent situation, such as the possibility that you have misunderstood the situation, retaliatory action from the perp and friends, police arriving on the scene thinking you're the attacker, etc.
I helped break up a fight at a bar a while ago, and it was only afterwards I thought, Oh, this really could have escalated to something serious. What if someone had a gun or a knife?
I totally agree that doing *nothing* is reprehensible. Absolutely. It's better, though, to be aware and prepared, than to rush in and act without considering possible and probable repercussions, ESPECIALLY when you bring a gun into the situation, which could include you going to jail. Drawing attention to the attacker, calling the police, getting backup from bystanders, these are all proper actions that don't necessarily include direct physical involvement.