It may well be she had a minor allergy to nuts, causing breathing problems, but no serious risk, just very uncomfortable.
Unfortunately that's a contradiction in terms.
I'm not sure it's a case of discrimination, more a question of reasonableness.
No more unreasonable than not having air traffic controllers with a stammer.
Personally, I'd always err on the side of the employee, but I'm not sure that it's safe to employ someone in such circumstances. I may be wrong.