Speaking as someone who does eat kosher, I can say it would be extremely important to me to be informed, whether I had eaten one bite or the entire pork hotdog and I'd be fairly pissed off at any friend who chose not to tell me out of some belief that it would protect me from some level of unpleasantness. I would imagine the same is true for vegetarians/vegans.
Most people choose to follow restrictive diets out of more than just moral belief systems. There is usually some practical side. For instance if you look at the list of non-kosher animals it generally includes predators and scavengers - animals who's meat is more likely to contain harmful chemicals and bacteria. It just makes sense to avoid that in my opinion. Sure, there is a religious component to my reasons for eating kosher, but I also honestly believe it is healthier.
A friend who knew I was eating something that wasn't kosher and didn't bother to tell me is obviously a friend who doesn't respect my point of view. That goes beyond not agreeing with it - that means not only do they not agree with it, but they think I'm an idiot for believing it in the fist place.....that's a lot to take.

