Although the best thing for the mother to have done was to find a real step-father, subjectively, I feel that this may have been beneficial. Little girls without father figures tend to be more likely to have behavioral problems, become pregnant in high school, drop out of high school, commit crime, engage in drugs and alcohol. It profoundly impacts child development, and it is easy to see who had a good father in their life and who did not.
Once the girl reached adulthood, however, they should have had the father "die" in some manner, without her ever having known.
If the mother had more appropriately found a partner to be with, it is quite possible that her new alternate step-father would not have really loved the daughter, either. As may be common, the step-father would always prefer his own children, and merely tolerate the other. She might not have had a real father either way.