Should you tell whom? The friend of your son, or your son? In either case, I don't think it matters that either of them need to know. What happens with you and that other woman don't need to be the knowledge of others. Of course your relationships with each person is unique and so is how you share things with each other. So it's still very much your call. I could see you telling your son, but the other woman should probably be the one to tell her own son.