What argument do you think the lawyers would make? A food establishment is supposed to be able to safely handle food. He requested food without an ingredient for health reasons and they agreed. Then they failed at food handling and he got sick.
It's a civil case, so the result can be a divided share of the blame. Something also tells me that they won't want to make the argument "no reasonable person would have any expectations that we got their order right".
Having a lawyer on retainer doesn't mean you're going to win, it just means you expect enough lawsuits to justify it. Recall the "absurd" McDonald's hot coffee case that 1) they lost despite having a lot of lawyers, and 2) wasn't absurd except through the lens of our society tending to label anyone suing a company as some combination of foolish and greedy.