I have a friend who slept with his best friend's widow?

Steer her towards the guy and tell her how much he wants to spend his time with her. She could be shutting herself out because she thinks people will judge her. Tell her its normal for her to see other people and go out more often and share her experience with someone who was close to her husband. But don't tell her you are aware of their relationship. If she tells you pretend you did not know.