Miss Swede has allergies to peanuts and tree nuts (except almonds) along with dairy, eggs, and sesame. When we board an aircraft, we immediately inform the purser that there is someone on-board with a peanut allergy. UA is usually very accommodating and will make an announcement in the vicinity of our seats. BA and SK are happy to make a full-plane announcement.
We always scan our surroundings. If someone pulls out a food item containing nuts, we politely ask them to refrain for eating it, then we offer to buy something from the BoB or give a snack from our supply. We've been very fortunate not to have any incidents.
The NRSA, in this case, should've informed the purser immediately upon boarding.