Most Helpful Guy
Talk to the b/f to make sure he's looped her in. For goodness sake do not leave it until you get there. Make absolutely sure. Or be prepared to partake whatever is served without complaint or comment.
Most Helpful Girl
I would ask him if he told her. Don't wait until you're already there, because she'll have already bought the supplies for dinner and likely started preparing it by that time. A vegetarian guest requires preparation.