How to cook for a diabetic without making the other guests suffer
I think the best solution is to avoid pre-mixing or pre-saucing anything. If you bring out a big bowl of pasta tossed (or topped) with sauce, the only way to get sauce is to eat pasta. Imagine instead you bring out:


a big bowl of pasta, perhaps tossed with a little oil to keep it from sticking
a bowl of brown rice
marinara sauce
alfredo sauce
olive-oil-grilled veggies (cauliflower, carrots, eggplant...)
grilled lean meat (chicken breast?)
mashed potatoes
a nice loaf of bread
a big green salad


Some people can have pasta and sauce. Some people can have meat and sauce. Some people can declare marinara sauce to be as close to a vegetable as they want, while others can eat veggies with rice or pasta and ignore the meat. If your alfredo is vegetarian and your marinara is vegan, you're covering a great deal of ground - diabetic, non-gluten, vegetarian, vegan - with one feast.

Personally I would avoid gluten-free pasta, or any kind of substitutions that are supposed to resemble something else. In a big dinner, you want to make it easy for people to know whether to avoid something or eat it. You also don't want people saying things like "ew, no, this is the fake pasta, please pass me the good stuff!" and making the person with a special diet feel bad.