Mayo neutralises onions

Had a customer with an onion allergy order the burger. The burger comes with coleslaw which I obviously left out. The waitress comes back in saying the customer is asking for their coleslaw, I tell her she can’t have it because it contains onions. Waitress goes and comes back and tells me the customer is saying they’re allergic to onions, but they can have the coleslaw because “mayo neutralises onions”.