Wow, that's an interesting topic and I've seen it handled successfully so many ways. We have our own take on this. So flame on...
We entertain a lot, several times each week for parties large and small. Special diet restrictions pop up all the time. Our general rule is to treat people in our homes in a reasonable manner, especially if that is how we would anticipate being treated in their home. To this end, we use two rules, the "health-benefit rule" and the "can we handle it" rule of thumb. The nightmare is of course, when you first learn of the guest's special need during the event; that could get more complicated but really it doesn't; it just feels different.
Case 1 - Culturally-related special diet law or cultural
preferenceThe rule of "Can we handle it ?" is applied. We are not a restaurant but we let guests know what the menu plan is ahead of time. If it happens to work out that they will or can eat what is offered, then great; but if there's nothing for them to eat and they choose to attend, then it simply an awkward mismatch of guest and party, one with which I am willing to live. Life is short. Those guests who limit their diet due to cultural preference are treated like everyone else; you get what we serve you unless we "can handle it". And we try to accomodate.
Some.
Case 2 - Health-related special diet requirements
We do prepare our food to at least partially accomodate guests whose special food needs we perceive to be truly health-related. We make a very bigger effort to adapt our menu plan for these people. So this includes folks following no-salt, low-salt, low-fat, low-choleserol, or one of various vegetarian or vegan diets. We handle similarly the cases of shellfish-allergy, peanut-allergy and citrus-allergy, at least when we know of the example ahead of time.
But for guests who are followers of one of the current fad "raw diets" or "Atkins diet", or "red meat aversion plans" or "pixie stix diet", since we have no belief that it presents humans with a definitive health benefit, we do not take extraordinary steps ahead of time to accomodate them. You communicate early, guest and host. You eat what we serve you, unless we can reasonably handle it another way. If we can handle it, sure, we try.
Some.
Case 3 - On the spot revelation of special diet needs
There is no special case 3 really. It's a result of poor communication, both guest and host. It always becomes a Case 1 or 2. It shouldn't happen to you as host, but when it does, do be of open mind, a big refridge, big pantry and a sous-chef. And try to help.
This serves us well. It's the communicate early and often part that helps most.
"No one can possibly know what is about to happen: it is happening, each time, for the first time, for the only time."
James A. Baldwin