My daughter has been on many a sleepover to a non-veg friends house (and one coming up next weekend). I always call the mom of the "invitor" and handle it sort of this way:
Me: Hi Mrs.So and so! I'm so glad you invited Daughter over for blah blah blah!
Her (usually): Oh, well... we Love having her over. It's been so long we thought the girls would like to do blah blah blah or go tum de dumming...
Me: I'm really glad they're going to do that! Sounds like so much fun, wish I could Blah blah blah, too. Ha ha ha!! Listen, I don't want you to go to any trouble with vegetarian food (you remembered that we're veggies, right?) Anyway, I'll send along some effortless stuff for her with enough to spare so everyone can try it. I'll make sure to send the whatchamawhoosits, because I know little so-and-so really loves them when daughter shares at lunch. And there so easy to make...if you like them I'll write it out for you.
Her (usually): Oh that's great. I'd forgotten Daughter was veggie!! I don't know if I would have thought about that before dinner. You don't have to send anything, I'll make sure we'll have a menu she can eat.
Me: Great. Thanks hope you have fun with them and thanks again for the invite!
(and then I let Daughter decide what she wants to take which is usually a ton of food. You know girls! That way if Mrs. So and so says "your mom didn't need to send "whatever", Daughter will say , "She didn't, I just brought munchies.....want some?"
Has worked like a charm and she's always invited back)