If they ask if he's allergic and will only remove it if is he is, then tell them yes, he's allergic to onion. It doesn't matter if his adverse reaction is uncomfortable or deadly, he can't eat it and that's all they need to know. You clearly got a salad scooped out of a big bin so it wasn't the freshest.

On occasion I've been surprised by how a menu item was prepared. One time in Idaho on a road trip I ordered French toast - pretty safe bet it's going to be bread, egg, and syrup. But no, they had a humongous piece of bread, dipped in an egg batter, then deep fried. I asked what on earth this was when it arrived and was told it's their own recipe. I should have sent it back. I tried eating it but it was quite greasy.

The weird surprise when I first moved to the South is that when eating breakfast out here if you order tea, they'll bring you iced tea unless you specify hot tea.