Cops have a duty to respond, as long as it won’t endanger our families or be too unsafe or something, even off duty. In my department at least it’s written into our standards of conduct, and not helping could be seen as cowardice which is unacceptable and grounds for discipline.
Now, of course this did wind up being too unsafe, but he made a judgement call. But you’re wrong about him not having authority while he’s not on duty. He’s not a civilian until he’s retired or fired, simply being off the clock doesn’t count.