There's a ton of veggie food out there, just takes a bit of looking. Check with your local health food store or large grocery stores with organic sections (you can quite often find meat substitutes there).
One thing I like, when I can find it is Fantastic Foods (I think they're based in California) Vegetarian Chili in a cup. Boil up some water, add to the cup, eat with or without crackers (all different kinds). Not only is it good, no mess to clean up afterward - toss the cup into the fire and lick the spoon clean.

I hate cooking outdoors anymore.
Another thing you can do before going is make up some veggie packs - sliced carrots, potatoes, onions and wrap in foil. Bake in the coals of the fire. Open it up, when done, add some margarine, salt and pepper and it's not too bad at all. A can of mixed vegetables, heated up by the fire can be ok as well. Remember things taste a lot better outdoors, than they do at home.
One thing you might like about this is no cooler, although if you take a cooler you can get all kind of meat substitutes - veggie hot dogs, burgers, etc.
If your girl friend is a vegetarian, I'm sure she must know things that can be taken. Have you actually tried talking to her about it?
Martoonisotu gave you a great suggestion - there is all kinds of things you can make with TVP. There should be recipes on line and also vegetarian cook books at the library. Experiment with things before camping. Even if you're a dyed-in-the-wool meat and potatoes guy, (remember, potatoes are a veggie!

) there will be some things you like.
My mother was a vegetarian all her life, my wife for the last 45 years or so, and I was for 6. One of these days I'm going back to it.
Anyway, good luck.
Doc