exactly because of allergies it is important to stick to the menu. if someone has a real allergy, not ‘i fart a little more’, they know exactly what they can and cannot eat. so they will order what they know is safe and the restaurant knows how to prepare the stuff on the menu.

i dont know how bad you can mess someone up with lactose, but milk has a large possibility of cross contamination. i know you can kill someone with celiac disease from just a little cross contamination. so lets just not risk it.