Help! a High Maintenance Plus-one!

This person should absolutely make her own preparations if her restrictions are that bad. You are not in the wrong and should not change your menu at all. You should not have given out your caterer's contact info - what if they harass the caterer so much that they decline doing the job? - that would be my fear. You are paying for the caterer and you should have sole contact with them - not these random a** people that you've never met. What if they change the ENTIRE menu?! Hopefully your caterer would have the sense to either not answer the questions or not change anything without talking to you first.

My FH's two nieces have very strict food restrictions as well, and their mother (my FSIL) will be bringing food special for them. She is not expecting us to make special accommodations for the girls (even though the venue said they would try their best, but after hearing how severe their allergies were they said it would be for the best if they brought their own food) - no big deal, she is used to providing food for her kids because she doesn't want to risk them potentially going to the hospital.

Bottom line, don't change anything - this woman needs to make and bring her own food if her allergies are that bad.

The caterer should be able to provide a special meal just for her. We had someone at our wedding who has a gluten intolerance and they provided a different plated meal for her. We also had someone who was a vegan and she also provided her own meal. Caterers typically are very understanding of allergies/dietary restrictions and are normally willing to make special accommodations. You shouldn't have to change your entire menu. She should be able to just get her own meal at no additional cost.

A significant other (spouse, boyfriend/girlfriend, common law spouse) is not a plus one.

Ignore these people and their complaints. Do not offer the caterer’s contact info. Normally on the reply card, you indicate for guests to let you know about any dietary restrictions so you can accommodate those (individually in the case of a plated meal or another dish labeled specifically in the case of buffet) . But it is out of line for either of them to ask you to revamp the menu or talk to the caterer. People like that want attention and don’t have genuine issues beyond their immaturity and lack of manners.

Exactly this.

I strongly disagree with this. The guest should not be expected to bring her own food, and that shouldn't be the solution - especially before the caterer has even heard what the request is and determined whether or not they can accommodate it.

I would give them the benefit of the doubt until they prove otherwise. They aren't actually asking you do to do anything other than give them your caterers' contact info. They aren't asking you to change your menus or make special accommodations. Maybe they literally just want to hear about the food ingredient and food handling practices from the source, and they have learned that going through middlemen (i.e., the wedding couple) doesn't get them the answers they need. They could also be ready to bring their own food once they learn what they can and cannot eat. They also might not have brought this up in the past 18 months because they know that menus aren't finalized until much later and that would have been counter-productive.

Everything in your OP suggests that you are primed not to like these people (starting with calling the wife of someone performing in the ceremony a "plus one"). I would just give your caterers a heads up, provide their contact information, and then not worry about this any more. Everyone involved already knows that no one can change the catering order but the people on the contract.

I suppose I read it as her trying to be accountable for her own dietary restrictions. By contacting the caterer herself, she'll know immediately if the ingredients in the options that you're serving are okay for her to eat. (I know that sometimes severe restrictions get lost in translation, and any issues she may have had in the past may have led her to just go straight to the source right off.) I would then assume that having the direct information would allow her to make an informed decision about whether she needs to bring her own food or not. I don't read her as being difficult, just careful.

You're hosting. This is literally your job, seeing to the comfort and safety of your guests. I feel like these people are doing their best to try and take this off your hands by contacting the caterer themselves. Some gluten allergies can be severe indeed. Sadly a lot of caterers are unaware of what actually contains gluten. I would feel better having contact with the caterer just to check. I believe they aren't asking you to change your whole menu, just to accommodate one person? At any rate, arranging something that your guest can eat is the polite thing to do.

I don't think this is the right solution. The guest should not have to bring their own food. It's easy enough to accommodate a gluten allergy if you know what you're doing.

Having an allergy does not make someone "difficult". That's not fair.