It doesn't say that he's anaphylactic though, just that he sought out medical treatment. I mean he could have been, but as far as I know anaphylaxis from onions is rather rare. Medical treatment could mean that he had diarrhea and got medication for that.

That being said, I wouldn't step into a burger place with an onion allergy. Especially since the onion allergene can be airborne. I have a soy allergy and you won't see me in an Asian restaurant.