I try to educate people as best I can on the proper care of fish, bettas included. Whether or not they take my advice is completely up to them.
Yes it makes me feel sick to the pit of my stomach when I see a betta languishing in a filthy bowl. But arguing with someone isn't going to do anything but make that individual more inclined to dig in their heels over it.
I simply prefer to lay out the facts as I did to LaLaLeyla (ammonia toxicity, stunting, nitrate build-up) and then let the person decide what course of action they want to take.