How far do you go to accommodate guests' diet?

I appreciate how much time and effort you have put into being a wonderful host! The kind of time and dedication it takes to accommodate dietary restrictions can be overwhelming, and I understand where you are coming from. I have Celiac disease and the thing I want to tell you is...it's okay to admit defeat! There will always be moments where circumstances are too much. In those moments, I think it is important to acknowledge two things: first, as host you are obligated to provide for your guests, and, second, if you can't do the first thing, you communicate in advance that you are unable to do so. So...how do you communicate you can't accommodate them? Personally. I prefer hosts to contact me directly and say something like, "I am having a very difficult time accommodating your needs at this event. These are the items I will be providing which fit into your diet." You can leave it there, or ask for help or recipe suggestions. Just, be kind and be clear. Give your guests as much information as you can, so that they can make the decisions they need to, to be their best, healthiest self.

Just a further note. Part of the purpose and appeal of a dinner party is the communion of eaters, enjoying the same meal.
If it's all buffet, there's no or less communion.
Maybe go the buffet route, but make a main dish/centerpiece that the guest(s) of honor can enjoy.
For example, in this case of a gluten and dairy avoiding honoree, a roasted cauliflower with, say, middle eastern spices, pomegranate seeds, herbs, sauces.
Or if cauliflower not to taste, something that is.
Then before and after everyone can have their own apps and desserts.

I agree that if hosting a party for a guest of honor with various self imposed restrictions then yes you do need to accommodate them- which doesn’t mean the entire menu does as well, just that there are options.
I’m vegetarian myself and have found the best strategy for multiple dietary needs is to have choices for guests- so instead of a plated composed meal serving family style and perhaps keeping a bacon garnish on the side, or having both potato gratin as well as dairy free roasted potatoes.

If this really is your best friend and this really does feel like an insurmountable menu change then have an honest conversation and say “I wanted to host your anniversary dinner but the more I plan it with multiple dietary issues to consider the more stressed out I get about the event- can we change our plan to celebrate at a restaurant instead?”

If it were me and I were entertaining for a special occasion, like your friend's anniversary, I would adhere to their dietary requests since the event is about them. If the dinner is next month, you have time to adjust your menu.

While it can be irritating to accommodate a growling list of dietary restrictions, I choose to view it as a fun challenge that I only do once in a while. I also think about the reason I'm offering to host a dinner party: is it for myself or someone else? And I'm not being preachy here! I have hosted parties purely for my own validation/ego/whatever - one time I hosted a party for the simple reason of wanting to show off the results of my new ice cream maker. But if the party is primarily for someone else, then their desires should be taken into account, in my opinion.

A guest of honor should always be accommodated. I never have a problem working with vegetarians, although my veggie friend who has become gluten intolerant is hard to work around sometimes. But if every other person has a restriction then it may be time to limit the guest list to fewer people. And a guest who offers to bring some diet special dish is a helper. (Even if it's not any good, at least you didn't make it!) But someone who insists on bringing a whole coursed meal is not someone I'd want to invite, after all what's the point? They are no longer really a guest.
Instead of an elaborate buffet with everything unassembled I'd go for a cocktail party. Lots of fun snacks and some good non-alcoholic drinks.

She gave you a month's notice. That's plenty and if you're hosting her, specifically, you definitely should accommodate her.

Part of hosting is making your guests comfortable. If you can't or won't do that, don't host. You don't know why they're restricting their food and they aren't obligated to tell you. Maybe whole30 or whatever happens to cover their myriad of food allergies or sensitivities. If you know, you're obligated to try or you shouldn't be hosting that person. I deal with my vegetarian in-law at Thanksgiving and my vegan or kosher-keeping friends. The other stuff feels more annoying because I think it's silly but if I care about this person and they warn me, I will respect their request. I would do the same if, for example, someone mentioned hating fish. I just ask for food restrictions when I send the invite.

If I invite someone over for dinner I do my very best to accommodate any dietary restrictions they have,whether they are self imposed or not. I would like to add that I do have a self imposed dietary restriction that is not a true allergy and I would never tell anyone it was an allergy because it is a sensitivity and not a true, life threatening allergy. Instead, I will often get a migraine, my psoriasis will flare up and I will be barely able to move my fingers for anywhere from 1 day to 1 week. My sister is a vegetarian and I've not heard anyone having an issue accommodating her self imposed dietary restrictions. I for one, never expect anyone to go out of their way to accommodate my dietary restrictions and will offer to bring my own food even. I think sometimes we need to take a step back and realize that while we may not understand why others have dietary restrictions, we should try to be sensitive to them. And, those of us with restrictions need to realize that others may not be able to accommodate those restrictions easily and try to help them out. For me this has meant inviting people to my place rather than going to theirs.

I have no problem accommodating diet restrictions stemming from a genuine medical need either. What gives me a slow burn is the same issue as you, namely that there are a plethora of voluntary diet restrictions out there now which seem to vary according the the direction of the wind. That's before we even touch on the whole business of "allergies" or "sensitivities" which are code for "I don't like/want to eat... whatever the food is. If you are the host, good manners dictate that you would naturally do your utmost to provide food and drink which were not a challenge or danger to your guest. On the other hand, a guest has certain obligations as well. Namely that unless there is a medical, or perhaps religious requirement involved, a guest should eat whatever is offered without passing remarks or objections. It unreasonable to expect people to be able to meet all of the current varied diet restrictions. I think the idea of offering a buffet is ideal, and could easily be built on a theme with a variety of dishes which could meet the most common limitations. That fulfills your obligations as a host. If your guest is not willing to fulfill their obligations as a guest, then that's their problem. So far as I am concerned, you offer your best, and they can decide to take it in that spirit, or they can reject it and go hungry. I suppose if it is a very close friend or relative, they could always bring whatever food suits them, and at least join everyone at the table without making a big deal of it. As far as what to do about your friend and a celebration, maybe you should ask her exactly what she would eat and go from there. Since the diet restrictions are voluntary, she might be willing to compromise in the spirit of celebration. If not, then it might be time to consider a get together not involving food.

Jennifer, are you asking the general questions you pose near the end or the more specific one of how to handle this anniversary party or both?
If the general, that opens up a variety of approaches.
If this party and you already told the guests of honor, you may have fewer or more limited options.
In the older threads on thid issue, a solution often suggested was a buffet that let people eat what they wanted and avoid what they didn't. A taco bar, a Dutch rice table, etc.
If that doesn't work, what about switching the occasion (another meal) or the type of get-together (not a meal)?