If I were you, I would ask your friend if he could speak to her (and hopefully let you speak to her too) and simply ask her exactly what she can eat. Hopefully her answer will give you an idea of something you could make, most likely a side dish, so that she will have something to eat.
Or if you’re really freaked out, you could ask your friend if he and his girfriend could possibly bring a dish that she could eat, because you don’t want to guess at what she might be able to eat and you don’t want to make her become ill.
The one thing on the list that you are unlikely to be able to fulfill, with one dish, is the fat-free part. Right now, I’m thinking some type of roasted chicken might work. You didn’t say whether or not she’s a vegetarian, but maybe you can find out. If she is, the chicken thing won’t work at all.
Here’s a recipe for Garlic Roasted Chicken that might work. Just make sure you don’t use any salt or salt-blend spice mixtures. Mrs. Dash makes a bunch of good spice mixes that don’t have salt in them.
And here’s a vegetarian, gluten free recipe that might work Portobello Mushrooms Stuffed with Quinoa just don’t use any salt for the seasoning.
And here’s a recipe for Baked Salmon leave out any salt, add a little bit of lemon juice instead.
I’ve made this Mushroom Quiche multiple times, with the fat free milk. You can omit any salt, and you can get a gluten-free frozen pie crust at Whole Foods or similar places.
But talk to them, right away, then you can get back to us and we can suggest more recipes. If the boyfriend doesn’t take you seriously, and says he doesn’t know, urge him to let you call her, yourself, because you simply have no idea of what she can eat and you want to do your best to try to accomodate her.