Whether you did the 'fix' for him or the law forced him to do the fix, I think that you run the risk of getting him really mad and doing some harm. Apparently, he's a non-conformist. Some people are that way.
You could still try the Municiple route to see, at least, if he's in some violation (w/o even having an Inspector go to him). And if he's not in violation, maybe still, you can ask that an Inspector tell him (for you), "No, Mr. Smith, you're not in violation of any code but what 'harm' would there be to turn your spout a different direction?" (Now I'm assuming that if he turned his spout, the water coming out the spout will do him no harm.) The police are not able to intervene because, well, no crime has been committed. AT worst, assuming you don't go the municiple or legal route, you can take the necessary measures on your parent's property to get rid of the water. Just don't do anything that will put it BACK on your neighbor's property because, again assuming he's not in violation, YOU'D be cited for a violation. In short, you can basically catch his water and dump it to the street or elsewhere on your lot. (Preferably, the street since volume is a concern.)
How 'bout talking to a neighbor that he pals with or knows. Go to them crying that your folks are in their 80's and have suffered damage that could have easily been prevented by turning a downspout. DO the drama because you're gonna be relying on someone to do your talking. Your folks may know who he knows on the street. It may be better to get 2 people to go to him TOGETHER because if they 'like' your folks, and your folks are 'good' to them, they'd go to bat as best they can. My best to ya and hope this helps. DO tell us if you have any luck. (YOu may have to e-mail me/us directly since your original post will get 'pushed down' on the Forum pretty quick.)