This whole thread reminds me of a time when I was eating something in the cafeteria at College (an ice cream) and a friend of mine whom I had not seen in a while, though she was also a student there, came up to me, grabbed the ice cream out of my hand and flung it in the garbage, saying, “Don’t you know this is not kosher?” BUT – SHE was mistaken. She had not yet heard that the company had in the last two months gone under a hechsher.
Meanwhile, I was embarrassed, she made a scene, I was out the yummy ice cream, and she did not offer to pay for it (and I didn’t demand that she do so, as people already were staring). So what’s my point?? I think in the end, people do want to (quietly) be made aware that something they are getting is not kosher, but be VERY sure that you are right. And above all, never embarrass someone who is eating something you think is treif. You could be mistaken, too. I would go to the person and say, “Oh that looks delicious; I didn’t know they finally got a hechsher!” That gets the point across, and then it is up to the person to take the message or not.