When someone suffers the loss of a spouse, many times they also lose their closest mutual friends. It's uncomfortable for most people because they knew them as a couple, once that changes people become uncomfortable, and do not know what to say. Please don't, as the remaining partner needs the friends now more than ever.
Step2
They don't understand the alienation and shouldn't have to go through it. Just keep calling once a week or so, Ask how they are and do they need anything.
Step3
As they heal, you will too, Ask them to lunch, What to say? What did you always talk about? Many times they welcome the invitation and are glad you are still there for them. I speak from experience when I say that when you lose a spouse, you shouldn't lose the whole circle of friends as well. So pick up that phone, or drop in for a visit!
Tips & Warnings
You can start off with just letting them talk, everything else will fall into place.