On this side of the pond, (given that the “allergy” diagnosis is very remote and, frankly, questionable, given the asymptomatic interim exposures, I would advise the applicant to undergo formal allergy testing by an allergy/immunology specialist prior to his aviation medical.
If he is negative, then the allergy was probably not ever real, but certainly isn’t an issue at present. A call to the regional flight surgeon wound almost undoubtably result in a ok to issue without further testing requirements.
If it’s positive, the powers that be will probably look askance at letting someone who is at risk for sudden debilitation be up front in any airplane with nothing but an epi-pen to keep him alive, much less an airliner.
https://www.faa.gov/about/office_org...amd/allergies/