Everyone has written such wonderful things. mamui good for you for trying to accentuate the positive for your friend. Please do listen to your friend and let her vent all feelings. One set of twins overwhelmed me I could not imagine two sets. She must be having some powerful feelings.
Offer to take care of her older twins. Give her time to nap during the pregnancy and bond with the babies afterwards. I found having people offer to do specific things was great because then I did not have to think about what they could do. Sometimes people just did things. For example my two friends came over at a prearranged time and did a load of laundry, cooked dinner and took my older daughter to the playground. I was involved in none of the decisions they "just did things". I sat on the couch and dozed/nursed the twins.
Hire someone to clean her house or do it yourself. What a wonderful feeling to have the whole house clean all at the same time. Even if it is just for 10 minutes.
I agree with the meals. We had meals for 3 weeks after the babies came home and it really helped. If she can't think of anything for you to do bring over food. Anything will do. Make up some tuna salad, egg salad,enchildadas, lasagna, chicken pot pies, pre made sandwiches,chili, stews anything that she can eat easily and quickly is ideal.
Offer to go to the grocery store for her. That could become very difficult for her with two sets of small baby twins. Think of the logistics!!!!!
Sometimes taking charge and just moving forward with something that you feel is needed is a relief. If she is your good friend she'll forgive you later for all the liberties you take. She will realize that you were trully helping. Example from my life was the day my really good friend took my older daughter home from school. I was so deeply asleep with the twins I did not hear the door bell ringing. After a few minutes my friend assessed the situation, left a note and took my daughter home to her house for the afternnon and dinner. When I woke up I was in a panic and really feeling awful/guilty. My daughter came home happy with her fun afternoon never having realized that I had "abandoned" her and I got some much needed sleep. My friend did the right thing. Even though given the choice I would not have gone that route. Thank goodness for really good friends.