I don't think you need to have an exact replica of everything else you're serving, but you absolutely do need to have something more substantial for her than veggies and dip. To me, it's not the fact that these are snacks that make them completely insufficient, but the fact that they have almost no calories.
The only substantial calories from veggies and dip come from the dip itself, which is going to be come combination of fat and sugar. If the only other thing you had that she could eat was ice cream, that would be more fat/sugar. Most people aren't going to feel too hot if they only have fat/sugar to nibble on for a few hours.
I think you have a number of options, but even something as simple as expanding the veggie tray to a full coldcuts tray (with some cheese, some meats, and some GF crackers) would do, if that's all you can manage. The protein and complex carbs there will make a world of difference.
In my opinion having an individual GF microwavable meal would be gracious and hospitable. It's not strictly necesssary, if you have *something* for her to eat that's more than veggies/dip/icecream, but it's certainly nice. People have done this for me as a vegetarian on a few occasions, and it always really nice.
Or you could ask your mom to bring over an individual GF meal of her choosing.