Originally Posted by MeefZah
To answer your question in the scenario you present, the citizen has no legal obligation to intervene. The LEOs have not asked him to assist, and even if they did, it's doubtful he could be legally compelled to act with a suspect running around shooting at him.
I agree with this. Of course, I am not 100% sure what the question is.
Are you (OP) asking if this person has a "duty" to render aid, or would be justified in rendering aid?
I would say they have no legal duty. I would say that given the strict confines of the "what if" they would be justified in rendering aid.