I'm so sorry to hear about your friend's husband. I understand what you mean about finding the right words to say. Maybe you could just drop her an "I'm thinking of you" card now and then. It doesn't need to say anything particular about the situation, but it will just let her know that she hasn't been forgotten. Maybe make a meal for her or dessert now and then as well. These "little" things can go a very long way in easing the pain!
Hugs to you and this friend!