There are lots of vegetarian choices in supermarkets, but my 'go to' choice would be vegetarian mince. You can use it for shepherd's pie, cottage pie, lasagne and all sorts of things like that. And most meat eaters won't mind eating veggie for a few days, especially if it makes life a litte easier for you.
Getting them to send you a list of suggestions would be good.
When I've cooked for coeliacs in the past I've not cooked their food separately, just cooked the same gluten-free things for everyone.
And get your DH to help!
Most of all remember they're coming to visit YOU, not the house or its contents, or even the food. You can always suggest they go out to eat!
What is a reasonable minimum spend for an online grocery delivery??