I would say that if someone has disclosed to you the pain level/duration of their allergy, there's some flexible judgement to be made. At least one of my friends is at painful hours on the toilet from one bread crumb, and we have (informally) negotiated that she wants a warning when she's about to get glutened.

And if someone is making their decisions about food based on personal ethics, the time to have a conversation about the ingredients is to have a meta-conversation outside of a food context (like, not at that picnic), where you ask them if they would appreciate you warning them about ingredients or if they've got more of a Don't Ask Don't Tell thing going on.