I don't know if this is normal or not, but many kids do it. So I wouldn't exactly think it was abnormal.
I think kids think about it less then we do and just want peace. Talking to their father may mean drama to them and no kid wants that.
Also, your friend's husband went to a dangerous country and could get killed. Big difference. But there is no doubt in my mind, they know far more then you are telling them and I am sure they have anger toward him. They aren't deaf, dumb and blind.....like AH.
Either way, they do need to talk about it so maybe you should find them all a good therapist.