It's not an easy situation, and airlines are often scumbags but I really don't think the airline is totally at fault or should be ashamed here. As much as it's an unfortunate disability, an accomodation for it does not have unlimited range to affect others' rights and liberties. There are several accommodations and alternatives, such as relocation to an alternate seat where no one is eating nuts, arranging private transportation (that is one of the few things private air travel is practical for).

Disability accommodation regulations even state that it has to be at a reasonable level of cost/effort for the person or company accomodating.

A close relative of mine has severe/life-threating shellfish allergies. I still don't think it would be a good idea to ban serving shellfish to other customers, but at most if it was on the menu, to be requested to relocate to somewhere with a lesser degree of exposure.