The advise to bring your own meal or buy one prior to departure is prduent, not just for the OP's fellow traveler, but anyone period. If someone has a food allergy, especially a severe one, there is no way to easily verify the contents. In this case, it's entirely possible both meals have mushrooms, even though the menu, online description, and even flight attendants would say only one does. So the solution here is to only eat food that you know how it was prepared. Don't trust the health of one individual to others.