Resolving problems and conflicts with friends is tough, but in cases like this I'd recommend something resembling a formal negotiation technique. And since I'm a bookish person, I recommend books rather than some ready-made solution.
Two books on the subject that I've found useful are Getting to Yes and also Crucial Conversations. I'll provide links to audio books.
**Getting to Yes** by Fisher and Ury
$1​
**Crucial Conversations** by Patterson, Grenny, McMillan, Switzler
$1 In a situation with friends, it may be helpful to have a third friend act as an intermediary. Also, it can help to focus on some objective standards and negotiate from there.
Whether or not a guide dog can be in the same room as someone with an allergy doesn't--or at least shouldn't--have to mean an either/or, all-or-nothing decision about a friend attending a wedding. Working on a creative solution together can be good because, hey, they're friends, and that's what friends do.
Could the friend and her guide dog listen to a live audio feed from a neighboring room? Hard to say without knowing more, and this is a more general concern that doesn't merit wading into the nearly 2000 comments on the original post.
Incidentally, if they're having a wedding now, in the midst of the COVID-19 pandemic, how are guests going to deal with travel and being close to one another anyway? "Intimate" doesn't have to mean crammed so close together you can smell the breath and/or deodorant of the person officiating the wedding. They may already be making accommodations, and in that spirit it seems that friends can find some way to be together but not close.