Ughhhhh....that is how vegetarians get served food with gelatin in it. And so forth. It isn't just the absence of meat/animal products. Do they eat cheese made with rennet? How do they prefer their protein? Do they eat tofu? TVP? Beans? Quinoa? Wheat?
I am not trying to be a jerk or knowitall....but this is exactly how a trip ends up 30 miles in with a person saying "I don't like this".
Frankly, most commercial vegetarian meals sold for hikers are ethnic in flavor. If they don't like Thai or Indian you could have issues. And worse is the heavy reliance on TVP in the meals. If they don't eat it at home they run a real risk of having severe stomach indigestion.