There's some kind of spicy green bean Chinese dish that probably is non dairy. Just make whatever veggies you were going to make, plain, and the non-allergic folks can add butter or whatever to theirs, while FIL eats them plain or wiht olive oil.
Tossed salad is easy and dairy free (no cheese, obviously). Don't put any dressing on it, let each person choose.
Appetizers--pickles, olives, raw veggies. Use a hummus dip if you want a dip (chickpeas and olive oil, flavored with garlic). Or toasted pita bread wedges with olive oil for dipping. Crackers and hummus. Pretzels. Fruits. Apples with caramel sauce (check labels for dairy, you might have to make your own).
Desserts--apple pie can be dairy free, probably pumpkin too if you use soy or almond milk instead of regular.
You are a nice DIL to do this :-) have fun, it will be OK!