NEVER! Its wrong & evil! Lying to a child about their father and pushing the father out of the child's life shows that you don't truly love your child. A good mother wouldn't do this! Stop Parental Alienation!
I will always tell them the truth when asked. And then I believe in them enough to make their own decision in the truth. They have thanked me many times for telling them the truth. And to be trusted by your children is even greater than to be loved.