One should be very careful while taking the responsibility of very young children, they miss the mother a lot. Certain things a child can confide in the ears of mother only. I think your friend should take the help of his wife's relatives for few years till the children absorb the facts.
As for me, my children were were above 18 years and on their own when their mother died.
But still now, my younger child aged 18, sometimes becomes very sad and would sit alone in corner of a room.
He is not brave enough to cry but I can see he has not become a man yet.