h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Although this article is excellent, it focuses entirely on the spiritual or ethical vegan. It is important to remember that not all those who stick to a strict vegetarian diet are doing so out of a moral stance. More and more people are making the choice to exclude animal products from their diet for reasons of health.

It is unclear whether science will validate some of the benefits that are claimed made for a vegan diet, nonetheless there are many people who feel strongly that such claims make enough sense that they are willing to make the effort. It is just common, simple courtesy to help them toward their goals or, at the very least, not get in their way.

It doesn't matter whether they can tell or not, whether it makes them sick or not, using any trace of animal products in food that you intend to serve to someone who believes that their life may depend on avoiding those ingredients is tantamount to an assault.

If you do not wish to comply with their dietary restrictions or, if for some reason you are incapable of doing so, then do not invite them to dinner.