This all reminds me of a current situation that we have. Every couple of weeks we get together, along with several others, at another family's home for a night of fellowship. It's slightly different than the situation in the OP-the hosts make a main meal and the rest of us bring sides or desserts. Well, three of the four of us in our household are or need to be on a carb-restricted diet, and hubby is super-picky. Which means that there are quite often very few "good" choices for us. Hubby is of special concern because he is diabetic, and if he doesn't eat it can get bad.
As far as I know, our hosts are still unaware that we are restricting carbs. It is no one's problem but our own. We eat what we can, restrict portions on choices that aren't as good, and I always bring a sugar-free low-carb cheesecake so we can have dessert. I'm very upfront with everyone that there are artificial sweeteners in my desserts, but that is as much fuss as I make. I always have snacks on hand for hubby (because I have the purse), so if he doesn't get enough to eat he doesn't fall over in shock.
This week our hosts are making lasagna. None of the four of us will probably like it-DD1 and I won't want the carbs, and DD2 and hubby are picky about the lasagna (they only like mine). And I will guarantee that our hosts will never notice. I'm certainly not going to tell them "we can't eat that". No. Just, no.
I think the friend in the OP is out of line if she really expects a separate meal to be made. I agree with the PP who suggested letting her express that rather than jumping in with a solution. And I think it's okay to say, "Oh, I'm sorry you won't be joining us." And if she is a really good friend, at a separate time saying that making two meals is too much is an okay thing to say.